Someone's mate is their spouse, partner, boyfriend, or girlfriend. Your grandmother's long-term sweetheart is her mate. Awwww.
A husband or wife is one kind of mate, and animals have mates too, chosen for reproduction and sometimes to assist in raising babies. Anything that comes in a pair — like socks, contacts, shoes, or earrings — also has a mate. Your pal or buddy can be called a mate as well. As a verb, to mate means "to breed" or "to reproduce." In Middle Low German, mate means "one eating at the same table."