a person who is frequently in the company of, associates with, or accompanies another

my son and his two companions.

a person employed to accompany, assist, or live with another in the capacity of a helpful friend.

a mate or match for something

White wine is the usual companion of fish.

a handbook or guide

a bird watcher's companion.

a member of the lowest rank in an order of knighthood or of a grade in an order.

Also called companion star , comes. Astronomy . the fainter of the two stars that constitute a double star. Compare primary ( def. 19b ) .

Obsolete . a fellow.

to be a companion to; accompany.

adjective

lone , lonely , lonesome , single , solitary , unaccompanied