Tracking pixel
A small piece of code embedded in a web page whose job is to report back to a third party that you visited it.
The name is a leftover from when these really were one-pixel images, invisible on the page, loaded from somebody else’s server so that the request itself was the message. Today it is usually a small script, but the principle is unchanged: an advertiser puts a snippet from an ad company on its pages, and when you load the page, your browser quietly tells that ad company you were there.
What makes a pixel powerful is not the visit itself but the identifier that travels with it. If your browser also sends a cookie tied to an account you are signed into, the ad company can connect this visit to that account rather than to an anonymous stranger. That is the whole mechanism behind cross-site advertising measurement, and it is why cookie settings and browser tracking protections matter more than they sound like they should.