What is Ally?

Legal Definition
International law. A power which has entered into an alliance with another power. A citizen or subject of one of the powers in alliance, is sometimes called an ally; for example, the rule which renders it unlawful for a citizen of the United States to trade or carry on commerce with an enemy, also precludes an ally from similar intercourse. 4 Rob. Rep. 251; 6 Rob. Rep. 406; Dane's Ab, Index, h. t.; 2 Dall. 15.
-- Bouviers Law Dictionary
Legal Definition
A nation which has entered into an alliance with another nation. 1 Kent, Comm. 69. A citizen or subject of one of two or more allied nations.
-- Black's Law Dictionary