1. One that is united or associated with another or others in an activity or a sphere of common interest, especially:

   1. A member of a business partnership.

   2. A spouse.

   3. A domestic partner. See Usage Note at domestic partner.

   4. Either of two persons dancing together.

   5. One of a pair or team in a sport or game, such as tennis or bridge.

2. Nautical A wooden framework used to strengthen a ship's deck at the point where a mast or other structure passes through it. Often used in the plural.

1. To make a partner of.

2. To bring together as partners.

3. To be the partner of.

Synonyms: These nouns all denote one who is united or associated with another, as in a venture or relationship. A partner participates in a relationship in which each member has equal status: a partner in a law firm.
A colleague is an associate in an occupation or a profession: a colleague and fellow professor.
An ally is one who associates with another, at least temporarily, in a common cause: countries that were allies in World War II.
A confederate is a member of a confederacy, a league, or an alliance or sometimes a collaborator in a suspicious venture: confederates in a scheme to oust the chairman.