(reciprocal pronoun) To one another; one to the other; signifies that a verb applies to two or more entities both as subjects and as direct objects:

(idiomatic, of two persons) Well suited to be in a relationship with one another, especially as romantic or marital partners.

(idiomatic) In mutual conflict.

(idiomatic, reciprocal) To compete with others eagerly or fiercely in pursuit of the same goal(s).

(UK, informal) To be found together most of the time.