A 1×1 pixel-sized transparent image that provides information about an ad’s placement. In many cases, a tracking pixel is used to notify an ad tracking system that either an ad has been served (or not served, in some cases) or that a specific Webpage has been accessed. 1×1 pixel tags on many websites that can track web surfers’ location and activities online, such as a registration or conversion. Some are powerful enough to know what a user types on a particular site. Also known as: beacon, web beacon, action tag, redirect, etc.