Source : Webster's Revised Unabridged Dictionary (1913)

Masturbation \Mas`tur*ba"tion\, n. [L. masturbatus, p. p. of
   masturbari to practice onanism: cf. F. masturbation.]
   Onanism; self-pollution.

Source : WordNet®

masturbation
     n : manual stimulation of the genital organs (of yourself or
         another) for sexual pleasure [syn: {onanism}, {self-abuse}]