A single image request to Adobe servers, generated when a user requests a resource on a website. A request can result in an error or a successful transmission of any data type. Each Track and Track Link call generates a hit.
About Hits and Visitor Data
When a user first visits your site, Adobe servers place a cookie containing a visitor ID value on the user's computer. Each image request that a user sends into Adobe's servers passes this visitor ID value back to Adobe. Because this value stays the same from hit to hit and from visit to visit (unless the user clears cookies), a series of hits containing the same visitor ID values can be construed to be a single visitor.
Using the time stamps of the hits recorded from each user, analytics can determine when the user made the first image request, and also when the user's inactivity on your site reached 30 minutes, which ends the visit. This process allows Adobe's servers to determine how many visits have been made by a single visitor ID value. The timestamp of each hit also allows reports to sequentialize the hits within a visit, making pathing data possible in reports.