Used other than figuratively or idiomatically: see best, friend. / An especially close and trusted friend. / (informal) An object or concept considered very useful or helpful.

A very close friend.

Dated form of boyfriend.

(often in the plural) Any of a group of friends who are often in each other’s company, and appear to be inseparable

(slang, dated, euphemistic) A homosexual man.

(physics, philosophy) The hypothetical observer in a thought experiment in which Wigner, absent when his friend observes the state of a particle (in some versions, Schrödinger's cat) as it collapses from quantum superposition, concludes that both the particle and his friend remain in quantum superposition until Wigner himself learns the result of the observation.

(UK) A designated friend or relative who collects medicine on behalf of a person with flu who has been advised to remain at home.

(slang) The situation in which one is viewed by a potential partner as a platonic friend, which complicates the initiation of a romantic relationship.

(slang, transitive) To classify as a friend, without any possibility of a romantic relationship.

A type of corkscrew having a folding design and including a small blade resembling a pocketknife