noun

1. A person, group, or state (etc) which is associated with another for a common cause; one united to another by treaty or common purpose; a confederate.

   “the two countries were allies in WWI”

2. A person, group, concept (etc) which is associated with another as a helper; a supporter; an auxiliary.

3. Anything akin to something else by structure, etc.

verb

1. To unite, or form a connection between, as between families by marriage, or between princes and states by treaty, league, or confederacy.

2. To connect or form a relation between by similitude, resemblance, friendship, or love.

Synonyms: make common cause