Ally, n.; pl. Allies.
Etymology: [See Ally, v.]
1. A relative; a kinsman. [Obs.] Shak.
2. One united to another by treaty or league; -- usually applied to sovereigns or states; a confederate. The English soldiers and their French allies. Macaulay.
3. Anything associated with another as a helper; an auxiliary. Science, instead of being the enemy of religion, becomes its ally. Buckle.
4. Anything akin to another by structure, etc.
2 other Webster entries under this spelling
Ally, v. t. [imp. & p. p. Allied; p. pr. & vb. n. Allying.]
1. To unite, or form a connection between, as between families by marriage, or between princes and states by treaty, league, or confederacy; -- often followed by to or with. O chief! in blood, and now in arms allied. Pope.
2. To connect or form a relation between by similitude, resemblance, friendship, or love. These three did love each other dearly well, And with so firm affection were allied. Spenser. The virtue nearest to our vice allied. Pope.
Note: Ally is generally used in the passive form or reflexively.
Ally, n.
See Alley, a marble or taw.
Shades of meaning
Fernald has no entry of its own for ally; he lists it under accessory, adherent, associate, auxiliary.
Where he writes about it:
Colleague is used always in a good sense, associate and coadjutor generally so; ally, assistant, associate, attendant, companion, helper, either in a good or a bad sense; abetter, accessory, accomplice, confederate, almost always in a bad sense. — from accessory
Ally is oftenest used of national and military matters, or of some other connection regarded as great and important; as, allies of despotism. — from accessory
An ally is more independent still, as he may differ on every point except the specific ground of union. — from adherent