The Collaborative International Dictionary of English v.0.48:

masturbation \mas`tur*ba"tion\, n. [L. masturbatus, p. p. of
   masturbari to practice onanism: cf. F. masturbation.]
   The act of masturbating; sexual self-gratification; onanism.
   [1913 Webster]

WordNet (r) 3.0 (2006):

masturbation
    n 1: manual stimulation of the genital organs (of yourself or
         another) for sexual pleasure [syn: masturbation,
         onanism]