An imaginary relationship that someone makes up in their head because they are secretly very deeply in love with someone else. It falls somewhere between crush and stalking.
John: "Man I wanna tell you something but you have to promise to keep it a secret."
Fred: "Sure thing man, whats up?"
John: "I'm totally in love with Kelly man!"
Fred: "Dude that's never gonna happen, I think you have an imaginationship going on."
John: "Yea I guess your right."