According to the International Council on Somnambulatory Coitus:
The act of forced sexual congress while one partner is asleep.
Contrary to popular opinion just having one partner asleep is not enough to constitute "slape." The partner must actively be resisting in a dream in order for "slape" to occur.
Andy: Ali stayed over last night.
Rick: Oh yeah? Anything happen?
Andy: Not according to her. but I seem to recall dropping a vicious load up in her delicious beefhole.
Rick: Isn't that illegal? That's rape!
Andy: Only in 49 states, my friend. I prefer to think of it as "slape".