Someone that you hang out with, you like but you wouldn't class as a friend and you spend too much time together to be just acquaintances. You're not colleagues or family and there is no sex involved.
Maybe you snogged at a party once but there was no spark or you bonded over your shared love of The Manchurian Candidate but that's where your commonality ends. You're comfortable grabbing a coffee with them when no-one else is available but would never dream of crying on their shoulder, discussing your hot and heavy date life or even letting slip where you live. They are an acquainta-friend.