Someone who was once a partner (any sense) but is not one any more.

(neologism) single (not involved in a romantic relationship)

(tennis) A player with whom one practises playing rallies, though not as a coach.

plural of partner dance

A person with whom someone has had sex.

a nominal partner

(intransitive) To form a partnership; to decide to work together in a duo.

A member of a civil partnership

(marketing) A business that generates demand for another business's advertisement inventory.

Domestic violence between people who are in an intimate relationship.