Measurement · also called Pixel, web beacon, tag
Tracking pixel
A tracking pixel is a tiny piece of code, historically an invisible 1x1 image, that loads when a page or ad is viewed and reports that event to a server.
When your browser loads a tracking pixel, it makes a request to someone's server, and that request is the message: "this page was viewed", "this ad was shown" or "this purchase happened". Originally it was literally a transparent 1x1 image; today it is usually a JavaScript tag that can also read and set cookies and collect more detail.
Pixels are everywhere in advertising. Ad servers use them to count impressions and clicks, verification vendors to measure viewability and invalid traffic, and advertisers to record conversions and build retargeting audiences. Social and search platforms each have their own advertiser pixel.
Pixels have become less reliable and more regulated. Ad blockers, browser tracking prevention, third-party cookie restrictions in some browsers and consent laws such as GDPR mean many do not fire or may not collect identifiers without consent. That is why advertisers now pair them with a Conversions API. Note that pixel stuffing, despite the name, is an unrelated ad-fraud technique.
Think of it like this
A tracking pixel is like a doorbell sensor that pings a distant control room every time someone walks in.
An example
An online bookstore in Germany places a conversion pixel on its order confirmation page; when a customer buys a €25 book, the pixel sends the order value and ad click ID to its advertising platform.
Related terms
Conversions API (server-side tracking)
A conversions API sends conversion events, such as purchases or sign-ups, directly from an advertiser's server to an ad platform, instead of relying only on browser pixels.
Third-party cookie
A third-party cookie is a small file set in your browser by a domain other than the site you are visiting, typically ad tech, letting it recognise your browser across sites.
Retargeting
Retargeting is showing ads to people who have already interacted with a brand, for example by visiting its website, using its app or adding a product to a cart without buying.
Attribution
Attribution is the process of assigning credit for a conversion, such as a sale, sign-up or install, to the ads, channels or touchpoints that a customer encountered beforehand.
Signal loss
Signal loss is the reduction in data available to target, measure and optimise advertising, caused by browser cookie blocking, mobile ID restrictions, privacy laws, consent choices and ad blocking.
Sources: Google Tag Platform: consent mode, Meta for Developers: Conversions API