Source : Webster's Revised Unabridged Dictionary (1913)

Onanism \O"nan*ism\, n. [Onan (--Gen. xxxviii. 9): cf. F.
   onanisme.]
   Self-pollution; masturbation.

Source : WordNet®

onanism
     n 1: manual stimulation of the genital organs (of yourself or
          another) for sexual pleasure [syn: {masturbation}, {self-abuse}]
     2: a method of birth control in which coitus is initiated but
        the penis is deliberately withdrawn before ejaculation
        [syn: {coitus interruptus}, {withdrawal method}, {pulling
        out}]