A person of the opposite sex that you hang out with a lot during working hours. They get jealous when you talk to others of the opposite sex, spend way too much time goofing off with you, etc., but there's no physical component to the relationship.
Coworker: Dude, are you hooking up with Allison from accounting?
Other coworker: I wish, but she's already married. She's just my office wife.