Privacy Sandbox glossary

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

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Ad platform (Adtech)

A company that provides services to deliver ads.


A company that pays to advertise its products.


Identification of user actions that contribute to an outcome.

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

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


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

Click-through rate (CTR)

The ratio of users who click on an ad, having seen it.

See also impression.

Click-through-conversion (CTC)

A conversion attributed to an ad that was 'clicked'.


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 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.

Differential privacy

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.


See Top-Level Domain and eTLD.

eTLD, eTLD+1

Stands for 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.


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.

Federated identity (federated login)

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

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.


Techniques to identify and track the behaviour of individual users.

Fingerprinting uses mechanisms that users aren't aware of and can't control. Sites such as Panopticlick and show how fingerprint data can be combined to identify you as an individual.

Fingerprinting surface

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).


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.

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.


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.


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


A 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).


Arbitrary number used once only in cryptographic communication.


Defined by the scheme (protocol), hostname (domain), and port of the URL used to access it.

For example:

Origin trial

Trials 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

Some 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.


In the Privacy Sandbox context, a site that displays ads.


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


Reaching people on other sites who have previously visited your site.

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


See Top-Level Domain and eTLD.


See Fingerprinting surface and Passive surface.


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.

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.

Note that some 'sites' are actually just subdomains. For example, and are subdomains of These subdomains are eTLD + 1.

User-Agent Client Hints (UA-CH)

Provide specific pieces of user-agent data on explicit request. This helps reduce passively exposed information which may lead to user identification or covert tracking.

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


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

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