Privacy Sandbox glossary

Privacy Sandbox articles and documentation assume a knowledge of concepts from privacy, advertising, and web development. This glossary explains key terms.

Ad auction (Protected Audience API)

In the Protected Audience API, an ad auction is run by a seller (likely to be an SSP or maybe the publisher itself), in JavaScript code in the browser on the user's device, to sell ad space on a site that displays ads.

Ad creative, creative

Ad creative refers to the contents of the ad served to users. Creatives can be images, videos, audio, and other formats. Creatives live within an ad space, and are served by ad tech within line items.

Ad exchange

An ad exchange is a platform to automate buying and selling of ad inventory from multiple ad networks.

Ad inventory, ad space

Ad inventory space is the space or spaces for ads that are available from a site that sells ad space.

Ad platform (Ad tech)

An ad platform is a company that provides services to deliver ads.


An advertiser is a company that pays to advertise its products.

Aggregatable reports

Aggregatable reports are encrypted reports sent from individual user devices. These reports contain data about cross-site user behavior and conversions. Conversions (sometimes called attribution trigger events) and associated metrics are defined by the advertiser or ad tech. Each report is encrypted to prevent various parties from accessing the underlying data.


Attestation is a mechanism to authenticate software identity, usually with cryptographic hashes or signatures. For the aggregation service proposal, attestation matches the code running in the ad tech-operated aggregation service with the open source code.


Attribution refers to the identification of user actions that contribute to an outcome.

For example, a correlation of ad clicks or views with conversions.

Blink is the rendering engine used by Chrome, developed as part of the Chromium project.


A buyer is a party bidding for ad space in an ad auction, likely to be a DSP, or maybe the advertiser itself. Ad space buyers own and manage interest groups.

Publishers make ad inventory available through marketplaces called ad exchanges, and buyers compete in real time through a DSP for the opportunity to place their advertisement.

Learn about ad space buyers in the Protected Audience API.


Chromium is an open-source web browser project. Chrome, Microsoft Edge, Opera and other browsers are based on Chromium.

Click-through conversion (CTC)

A click-through conversion is a conversion attributed to an ad that was clicked.

Click-through rate (CTR)

The click-through rate is the ratio of users who click on an ad, having seen it.

See also impression.


A conversion is the completion of some desired goal following action by a user.

For example, a conversion may occur with the purchase of a product or sign-up for a newsletter after clicking an ad that links to the advertiser's site.

A cookie is a small piece of textual data that websites can store on a user's browser. Cookies can be used by a website to save information associated with a user (or a reference to data stored on the website's backend servers) as the user moves across the web.

For example, an online store can retain shopping cart details even if a user is not logged in, or the site could record the user's browsing activity on their site. See First-party cookie and Third-party cookie.


A coordinator is an entity responsible for key management and aggregatable report accounting. The coordinator maintains a list of hashes of approved aggregation service configurations and configures access to decryption keys.

Coarse data

Course data refers to limited information provided by Attribution Reporting API event-level reports. This is limited to 3 pieces of conversion data for clicks and 1 piece for views. Specific, granular conversion data (such as specific prices of items and timestamps) are not included.

Data management platform (DMP)

A data management platform is software used to collect and manage data relevant for advertisers. These platforms help advertisers and publishers identify audience segments, which can then be used for campaign targeting.

Learn more about DMPs.

Demand-side platform (DSP)

A demand-side platform is an ad tech service used to automate ad purchasing. DSPs are used by advertisers to buy ad impressions across a range of publisher sites.

Differential privacy

Differential privacy refers to techniques to allow sharing of information about a dataset to reveal patterns of behaviour without revealing private information about individuals or whether they belong to the dataset.


Domain. See Top-Level Domain and eTLD.


Entropy, in the privacy domain, is a measure of how much an item of data reveals individual identity.

Data entropy is measured in bits. The more that data reveals identity, the higher its entropy value.

Data can be combined to identify an individual, but it can be difficult to work out whether new data adds to entropy. For example, knowing a person is from Australia doesn't reduce entropy if you already know the person is from Kangaroo Island.


In the Topics API, an epoch is the period of time during which the browser infers topics for a user based on their browsing activity. It is currently set to one week.

eTLD, eTLD+1

eTLDs are effective top-level domains (TLD), which are defined by the Public Suffix List.

For example:

Effective TLDs are what allow to be a different site from The eTLD in this case is, and the whole site name (, is known as the eTLD+1.

See also Top-Level Domain.

Event-level report

Event-level reports associate a particular ad click or view (on the ad side) with data on the conversion side. To preserve user privacy by limiting the joining of user identity across sites, conversion-side data is very limited, and the data is noisy.

Federated Credential Management API (FedCM)

Federated Credential Management API is a proposal for a privacy-preserving approach to federated identity services. This will allow users to log into sites without sharing their personal information with the identity service or the site.

FedCM was previously known as WebID, and is still in development in the W3C.

Federated identity (federated login)

Federated identity is a third-party platform to allow a user to sign in to a website, without requiring the site to implement their own identity service.

Fenced frame

A (<fencedframe>) is a proposed HTML element for embedded content, similar to an iframe. Unlike iframes, a fenced frame restricts communication with its embedding context to allow the frame access to cross-site data without sharing it with the embedding context.

Some Privacy Sandbox APIs may require select documents to render within a fenced frame. Learn more about the Fenced Frames proposal.


Fingerprinting encompasses techniques to identify and track the behaviour of individual users.

Fingerprinting uses mechanisms that users aren't aware of and can't control.

Fingerprinting surface

A fingerprinting surface is something that can be used (probably in combination with other surfaces) to identify a particular user or device.

For example, the navigator.userAgent() JavaScript method and the User-Agent HTTP request header provide access to a fingerprinting surface (the User-Agent string).


First party refers to resources from the site you're visiting.

For example, the page you're reading is on the site and includes resources requested from this site. Requests for those first-party resources are called 'first-party requests'. Cookies from stored while you're on this site are called first-party cookies.

See also Third-party.

A first-party cookie is a cookie stored by a website while a user is on the site itself.

For example, an online store might ask a browser to store a cookie in order to retain shopping cart details for a user who is not logged in. See also Third-party cookies.


Intent to Experiment (I2E) is the announcement of a plan to make a new Blink feature available to users for testing, typically through an origin trial.


Intent to Extend Experiment (I2EE) is an announcement of a plan to extend the duration of an origin trial.


Intent to Prototype (I2P) is the first stage in developing a new feature in Blink. The announcement is posted to the blink-dev mailing list with a link to the proposal for discussion.


Intent to Ship (I2S) is an announcement of a plan to make a new feature of Blink available to users in stable versions of Chrome.


Impression could refer to either:

  • View of an ad. See also click-through rate.
  • An ad slot: the HTML markup (usually <div> tags) on a web page where an ad can be displayed. Ad slots constitute inventory.

Interest group

In the Protected Audience API, formerly FLEDGE, an interest group represents a group of people with a common interest, corresponding to a remarketing list.

Every interest group has an owner. Different types of owners will create different types of interest groups with different use cases.


Inventory is the ad slots available on a site. Ad slots are the HTML markup (usually <div> tags) where ads can be displayed.


K-anonymity is the measure of anonymity within a data set. If you have k anonymity, you can't be distinguished from k-1 other individuals in the data set. In other words, k individuals have the same information (including you).


A nonce is an arbitrary number used only once in cryptographic communication.


An origin is defined by the scheme (protocol), hostname (domain), and port of the URL used to access it.

For example:

Origin trial

Origin trials are trials that provide access to a new or experimental feature, to make it possible to build functions that users can try out for a limited time before the feature is made available to everyone.

When Chrome offers an origin trial for a feature, an origin can be registered for the trial to allow the feature for all users on that origin, without requiring users to toggle flags or switch to an alternative build of Chrome (though they may need to upgrade). Origin trials allow developers to build demos and prototypes using new features. The trials help Chrome engineers understand how new features are used, and how they may interact with other web technologies.

Find out more: Getting started with Chrome's origin trials.

Passive surface

Passive surfaces are fingerprinting surfaces—such as User-Agent strings, IP addresses, and Accept-Language headers—that are available to every website, whether the site asks for them or not.

Passive surfaces can easily consume a site's privacy budget.

The Privacy Sandbox initiative proposes replacing passive surfaces with active ways to get specific information, for example using Client Hints a single time to get the user's language rather than having an Accept-Language header for every response to every server.

Protected Audience API

The Protected Audience API is the new name for the FLEDGE API.


In the Privacy Sandbox context, a publisher is a site with ad space that is paid to display ads.


Reach represents the total number of people who see an ad or who visit a web page that displays the ad.

Real-time bidding (RTB)

Real-time bidding refers to an automated auction for buying and selling ad impressions on websites, completed during page load.


Remarketing is the practice of advertising to people who've already visited your site on other sites.

For example, an online store could show ads for a toy sale to people who previously viewed toys on their site.

Reporting origin

The reporting origin is the entity that receives aggregatable reports—in other words, the ad tech that called the Attribution Reporting API. Aggregatable reports are sent from user devices to a well-known URL associated with the reporting origin.

Scaling factor

Scaling factor, in the context of the Attribution Reporting API, refers an amount by which you choose to multiply an aggregatable value. Scaling impacts the effect of noise and your contribution budget.


A seller is the party running an ad auction, likely to be an SSP or maybe the publisher itself.


A site is equivalent to an eTLD+1 along with a scheme (protocol).

See also Top-Level Domain.

Storage partitioning

Storage partitioning is a mechanism in Chrome to prevent certain types of side-channel cross-site tracking, Chrome is partitioning storage and communications APIs in third-party contexts.

Storage partitioning prevents a site from joining data across different sites to track the user across the web.

Summary report

A summary report is an Attribution Reporting API and Private Aggregation API report type. A summary report includes aggregated user data and can contain detailed conversion data, with noise added. Summary reports are made up of aggregate reports.

Supply-side platform, Sell-side platform

A supply-side platform is an ad tech service used to automate selling ad inventory. SSPs allow publishers to offer their inventory (empty rectangles where ads will go) to multiple ad exchanges, DSPs, and networks. This enables a wide range of potential buyers to bid for ad space.


Surface. See Fingerprinting surface and Passive surface.

Third party refers to resources served from a domain that's different from the website you're visiting.

For example, a website might use analytics code from (via JavaScript), fonts from (via a link element) and a video from (in an iframe). See also First-party.

A third-party cookie is a cookie stored by a third-party service.

For example, a video website might include a Watch Later button in their embedded player to allow a user to add a video to their wishlist without forcing them to navigate to the video site.

See also First-party cookie.

Top-level domain (TLD)

Top-level domains such as .com and .org are listed in the Root Zone Database.

See also eTLD, site.

Trusted Execution Environment (TEE)

A trusted execution environment is a special configuration of computer hardware and software that allows external parties to verify the exact versions of software running on the computer. TEEs allow external parties to verify that the software does exactly what the software manufacturer claims it does—nothing more or less.

To learn more about TEEs used for the Privacy Sandbox proposals, read the Protected Audience API services explainer and the Aggregation Service explainer.

User-Agent Client Hints (UA-CH)

User-agent client hints provide specific pieces of the User-Agent string on explicit request. This helps reduce passive surfaces in the User-Agent string which may lead to user identification or covert tracking.

UA-CH is sometimes referred to as "Client Hints."

User-Agent string

A user-agent string is an HTTP header used by servers and network peers to request identifying information about an application, operating system, vendor, or version of a user agent. The User-Agent string broadcasts a large string of data, which is problematic for user privacy. User-Agent reduction is proposed to remove sensitive information and reduce passive fingerprinting.


.well-known is a file used to add redirects to a website from standardized URLs.

For example, password managers can make it easier for users to update passwords if a website sets a redirect from /.well-known/change-password to the change password page of the site.

In addition, it can be useful to access policy or other information about a host before making a request. For example, robots.txt tells web crawlers which pages to visit and which pages to ignore. IETF RFC8615 outlines a standardized way to make site-wide metadata accessible in standard locations in a /.well-known/ subdirectory.

See a list of recommendations for .well-known at


A worklet allows you to run specific JavaScript functions and return information back to the requester. Within a worklet, you can execute JavaScript but you cannot interact or communicate with the outside page.

Worklets are used to store and extract data with the Shared Storage API.