A tracking pixel is a 1×1 transparent image embedded on a web page or in an email that fires a request to a tracking server when loaded — recording an impression, page view, or conversion event. The pixel URL carries user identifiers, session data, and event metadata as query parameters.
Pixels are the traditional client-side tracking mechanism for affiliate conversions and ad campaign measurement. They are increasingly unreliable due to browser ad blockers, iOS privacy changes (ITP), and third-party cookie deprecation. Server-side postbacks and Meta's Conversion API (CAPI) are gradually replacing pixels for conversion attribution in programs that need high accuracy.