a person, group, or nation that is associated with another or others for some common cause or purpose: Canada and the United States were allies in World War II.
5.
Biology. a plant, animal, or other organism bearing an evolutionary relationship to another, often as a member of the same family: The squash is an ally of the watermelon.
6.
a person who associates or cooperates with another; supporter.
Origin: 1250–1300; Middle English alien < Anglo-French al(l)ier, aillaier,Old French alier < Latin alligāre to bind to. See alloy
c.1300, from O.Fr. alier "combine, unite," from a differentiated stem of aleier (source of alloy), from L. alligare "bind to" (see alloy). The noun is c.1600 in the sense of "one united with another by treaty or league," from the verb.