Definitions for ATP
Random House Webster's College Dictionary
adenosine triphosphate: a nucleotide that is the primary source of energy in all living cells because of its function in donating a phosphate group during biochemical activities; composed of adenosine, ribose, and three phosphate groups and formed by enzymatic reaction from adenosine diphosphate and an orthophosphate.
Ref: Compare ADP.
Origin of ATP:
adenosine triphosphate, ATP(noun)
a nucleotide derived from adenosine that occurs in muscle tissue; the major source of energy for cellular reactions