Definition of ally noun from the Oxford Advanced American Dictionary

Oxford3000

ally

noun
 
/ˈælaɪ/
 
plural allies1 [countable] a country that has agreed to help and support another country, especially in case of a warour European/NATO alliesa loyal ally of the United Kingdom2 [countable] a person who helps and supports someone who is in a difficult situation, especially a politiciana close ally and friend of the prime ministerHis sister was his ally against their grandparents.3 the Allies [plural] the group of countries including the U.S. and Britain that fought together in World Wars I and IIthe Western Allies