"A variable, transient peak sensation of intense pleasure, creating an altered state of consciousness, usually with an initiation accompanied by involuntary, rhythmic contractions of the pelvic striated circumvaginal musculature, often with concomitant uterine and anal contractions, and myotonia that resolves the sexually induced vasocongestion and myotonia, generally with an induction of well-being and contentment."
~ C M Meston, R J Levin, M L Spiski, E M Hull and J R Heinman
Here's a couple quick definitions to help you out.
Motonia - muscle relaxation after contraction
Vasocongestion - variation in local blood supply
So ... any guesses?