A relationship where a guy and a girl who like each other spend every waking moment together but refuse to admit or agree that they are dating.
Q: I see Dave and Katie* everywhere together... in the library, in the student union, swing dancing... Are those two dating?
A: Maybe, but they won't admit it. Right now they're just Hillsdating. If you see them holding hands that means it's official.
*2 most common names at Hillsdale College.