The Collaborative International Dictionary of English v.0.48:

masturbate \masturbate\ v. i. [imp. & p. p. masturbated; p.
   pr. & vb. n. masturbating.]
   To achieve sexual gratification by stimulating one's own
   sexual organs, without the aid of a partner; -- typically to
   the point of orgasm. -- masturbator, n.

   Syn: wank, she-bop, jack off, jerk off, whack off, beat the
        meat.
        [WordNet 1.5]