When you have a crush on someone but there is one or more reasons why you can't date them, don't think it is worth trying to date them or not worth trying to get them to like you back. Usually occurs when you like someone you can't have.
In which case, most people would say, "I would like him/her if _____."
Jon: "So you hooking up with anyone?"
James: "Well, Emily is hot and I would hook up with her if she didn't have a boyfriend already."
Jon: "Sounds like you have a conditional crush."