Definition of: ally

(ə·lī′) v. al·lied, al·ly·ing v.t.
To unite or combine in an affinity, marriage, or association, by relationship, similarity of structure, treaty, or compact: generally in the passive with to or with, or used reflexively: The minister hoped to ally France with Spain against England; He allied himself with all liberal thinkers.
—v.i. To enter into alliance; become allied.
—noun (al′ī, ə·lī′) plural al·lies
1. A person or thing connected with another, usually in some relation of helpfulness or kinship; a state, sovereign, or chief leagued with another, as by treaty or common action.
2. Any friendly associate or helper.
3. An organism or substance associated with another by similarity of structure or properties. See synonyms under ACCESSORY, ADHERENT, ASSOCIATE, AUXILIARY.
—the Allies
1. The twenty-seven nations allied against the Central Powers in World War I; specifically, Russia, France, Great Britain, Italy, and Japan, adhering to the Declaration of London; the Allied and Associated Powers included twenty-two other nations, co-belligerent but not adhering to the Declaration.
2. The nations and governments-in-exile known as the United Nations in World War II. [<OF alier <L alligare < ad- to + ligare bind]