The Collaborative International Dictionary of English v.0.48:

orgasm \or"gasm\ ([^o]r"g[a^]z'm), n. [F. orgasme; cf. Gr.
   'orgasmo`s a kneading, softening, prob. confused with 'orga^n
   to swell, especially with lust; to feel an ardent desire.]
   (Physiol.)
   Eager or immoderate excitement or action; the state of
   turgescence of any organ; erethism.
   [1913 Webster]

   2. Especially: The peak of the physical pleasurable sexual
      excitement caused by stimulation of the sexual organs, as
      in intercourse; in the male, it is usually accompanied by
      ejaculation.
      [1913 Webster +PJC]

   3. Hence: An eruption of intense, uncontrolled excitement; --
      used of groups and individuals.
      [PJC]

WordNet (r) 3.0 (2006):

orgasm
    n 1: the moment of most intense pleasure in sexual intercourse
         [syn: orgasm, climax, sexual climax, coming]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "orgasm":
   abandon, access, act of love, adultery, aphrodisia, apoplexy, ass,
   attack, balling, carnal knowledge, climax, cohabitation, coition,
   coitus, coitus interruptus, commerce, congress, connection,
   convulsion, copula, copulation, coupling, cramp, craze, delirium,
   diddling, eclampsia, ecstasy, epilepsy, epitasis, falling sickness,
   fire and fury, fit, fornication, frenzy, furor, furore, fury, grip,
   hysteria, ictus, intercourse, intimacy, intoxication, lovemaking,
   madness, making it with, marital relations, marriage act, mating,
   meat, onanism, orgy, ovum, pareunia, paroxysm, passion,
   procreation, rage, rapture, ravishment, relations, screwing,
   seizure, sex, sex act, sexual climax, sexual commerce,
   sexual congress, sexual intercourse, sexual relations,
   sexual union, sleeping with, spasm, sperm, stroke, tearing passion,
   throes, towering rage, transport, venery