Kinda vague question, lol. Could be a definition for anything - in what context did you find it? A search on Google got me the following results:
AppleTalk Transaction Protocol. Transport-level protocol that provides a loss-free transaction service between sockets. The service allows exchanges between two socket clients in which one client requests the other to perform a particular task and to report the results. ATP binds the request and response together to ensure the reliable exchange of request-response pairs.
Advanced Technology Program. A cost-sharing technology development program of the National Institute of Standards & Technology (NIST). Allows the Federal government to fund high-risk research in general and focused program competitions.
Attach Point. This is the point where new polygons will be started from. Many tools also perform their operations around the ATP. In a new drawing, the ATP defaults to co-ordinates 0,0,0. After creating a polygon, the ATP moves to the co-ordinates of the last point created. If a polygon is selected then de-selected, the ATP will move to the point on the polygon that was selected.
Adenosine triphosphate, is a major energy storing compound. When ATP breaks down into ADP (adenosine diphosphate) and phosphate, it releases energy used to fuel many biochemical processes, such as muscle contraction, nerve action, protein synthesis etc. ATP is made, mostly in mitochondria, from ADP and phosphate. Food is "burned" under controlled conditions, by reacting it with oxygen from air, and the energy so produced harnessed to make ATP.
Hope one of those is what you're looking for.