'ally' definitions:

Definition of 'ally'

From: WordNet
noun
A friendly nation
noun
An associate who provides cooperation or assistance; "he's a good ally in fight" [syn: ally, friend] [ant: enemy, foe]
verb
Become an ally or associate, as by a treaty or marriage; "He allied himself with the Communists"

Synonyms of 'ally'

From: Moby Thesaurus