In biology, "ATP" stands for "adenosine triphosphate". ATP is a coenzyme that cells use for energy storage. ATP is present in every cell's cytoplasm and nucleus because it is necessary for essential life functions in plants and animals.
In animals, ATP is a product of glycolysis in the mitochondria, while in plants, it generally comes from photophosphorylation during photosynthesis. When used, ATP converts to adenosine diphosphate, or ADP, a chemical that can then be recharged with energy to become ATP again. Because of this, an organism's supply of ATP + ADP is constant, although the relative balance of the two fluctuates according to the organism's energy level.