A person who is thought of as more of a stand in or a temporary solution then someone who they meant to be hanging out with but is very reliable.

A good friend who is merely in love, but decides to help you with your love anyways. They're usually sweet and kind, but discreet. They're just forging a friend relationship with you to keep tabs in a non-stalker way.

An informal, at best verbal, understanding that what is being discussed is in confidence. No paperwork, just trust, protects ideas from being stolen.

See related term friendo
A friend of the female gender. The English language had previously made it too easy for women to make their boyfriend or would-be suitor jealous by telling him she's "out with a friend". Back then, for the next several hours after receiving that "with a friend" text from her, his mind would go from convincing himself that the friend is a girl from her pilates class, to thinking that this "friend" is a 6'3" surgeon taking her out on his 75 ft yacht. And back and forth between those thoughts. In reality, it was her childhood friend, Sarah, that she was with, but she knew what she was doing when she said to him "a friend" just to drive him to insanity in a seemingly innocent and unintentional way. Because, of course, if he had asked the sex of this friend, he would appear to be a jealous psycho. And he is still trying to play it cool.

Two people who are friends but act like they're dating

Used by males to distinguish a female friend whom of which the relationship with is platonic. To be used in lieu of saying "My friend who's a girl". Replaces the annoying need to stop a story and explain that the friend is not a girlfriend.
Similar to the Spanish word "amiga".

See also: friendo