verb, -lied, -ly⋅ing, noun, plural -lies.| 1. | to unite formally, as by treaty, league, marriage, or the like (usually fol. by with or to): Russia allied itself to France. |
| 2. | to associate or connect by some mutual relationship, as resemblance or friendship. |
| 3. | to enter into an alliance; join; unite. |
| 4. | 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. |