Illustrated Glossary of Organic Chemistry

Phosphate group: A functional group characterized by a phosphorus atom bonded to four oxygen atoms (three single bonds and one double bond). One of these oxygen atoms must be bonded to another atom; if not, the structure is a phosphate ion.

Generic phosphate group molecular structure.
Phosphoric acid

Adenosine monophosphate (AMP; adenylic acid)   
Adenosine triphosphate (ATP)