A number of things resembling one another, or belonging together; a set; as, a pair or flight of stairs. "A pair of beads." Chaucer. Beau. & Fl. "Four pair of stairs." Macaulay. [Now mostly or quite disused.]

Two things of a kind, similar in form, suited to each other, and intended to be used together; as, a pair of gloves or stockings; a pair of shoes.

Two of a sort; a span; a yoke; a couple; a brace; as, a pair of horses; a pair of oxen.

A married couple; a man and wife.

To be joined in pairs; to couple; to mate, as for breeding.

A male and female fish

When referring to animals this always means a male and female.

make love; "Birds mate in the Spring"

a list if its back is a list

a match or correspondence

a partial solution to the distributional Post correspondence problem with list if is a prefix of , and and are the concatenation of corresponding strings of -list and -list respectively