A way to track user data. Originally, all pixels were literally snippets of code that called for a 1X1 transparent pixel to be delivered to a webpage by a third-party server. Nowadays, pixels may either be literal pixels or be javascript based. When the pixel loads, the third-party server can record information such as the IP address of the user's computer, URL of the page, and time the page was viewed. See also conversion pixel and segment pixel.