The Collaborative International Dictionary of English v.0.48:

Extinction \Ex*tinc"tion\, n. [L. extinctio, exstinction: cf. F.
   extinction.]
   1. The act of extinguishing or making extinct; a putting an
      end to; the act of putting out or destroying light, fire,
      life, activity, influence, etc.
      [1913 Webster]

   2. State of being extinguished or of ceasing to be;
      destruction; suppression; as, the extinction of life, of a
      family, of a quarrel, of claim.
      [1913 Webster]

   3. Specifically: The ceasing to exist of a species of living
      organism, such as a plant or animal, whose numbers
      declined to the point where the last member of the species
      died and therefore no new members of the species could
      ever again be born.
      [PJC]

   Note: Extinctions have occurred many times throughout the
         history of life on Earth, and abundant evidence of the
         prior existence of animals and plants are found as
         fossils in rock formations many millions of years old.
         It is believed by some that due to the influence of man
         on the environment and destruction of habitat, the rate
         of extinction of species is now higher than at any
         previous time on this planet. Extinctions of some
         animals in recent years have actually been reliably
         recorded, such as that of the dodo bird. A remarkable
         example of extinction is that of the passenger pigeon
         (Ectopistes migratorius) in North America, which once
         numbered in the billions, and the last living member of
         which species was recorded as dying in captivity in
         1914.
         [PJC]

WordNet (r) 3.0 (2006):

extinction
    n 1: no longer active; extinguished; "the extinction of the
         volcano"
    2: no longer in existence; "the extinction of a species" [syn:
       extinction, defunctness]
    3: the reduction of the intensity of radiation as a consequence
       of absorption and radiation
    4: complete annihilation; "they think a meteor cause the
       extinction of the dinosaurs" [syn: extinction,
       extermination]
    5: a conditioning process in which the reinforcer is removed and
       a conditioned response becomes independent of the conditioned
       stimulus [syn: extinction, experimental extinction]
    6: the act of extinguishing; causing to stop burning; "the
       extinction of the lights" [syn: extinction,
       extinguishing, quenching]

Moby Thesaurus II by Grady Ward, 1.0:

118 Moby Thesaurus words for "extinction":
   abolishment, abolition, abscission, amputation, annihilation,
   annulment, bane, biological death, blackout, blocking, burning out,
   burnout, cessation of life, choking, choking off, clinical death,
   controlling, crossing the bar, curtains, damping, death,
   death knell, debt of nature, decease, dematerialization, demise,
   departure, deracination, destruction, disappearance, disappearing,
   dispersion, dissipation, dissolution, dissolving, doom, dousing,
   dying, ebb of life, eclipse, elimination, end, end of life, ending,
   eradication, erasure, eternal rest, evanescence, evaporation,
   excision, exclusion, exit, expiration, extermination,
   extinguishment, extirpation, fadeaway, fadeout, fading,
   final summons, finger of death, fire fighting, flame-out, going,
   going off, going out, grave, hand of death, jaws of death, knell,
   last debt, last muster, last rest, last roundup, last sleep,
   leaving life, liquidation, loss of life, making an end, melting,
   mutilation, negation, nullification, occultation, parting, passing,
   passing away, passing over, perishing, purge, putting out,
   quenching, quietus, release, rescission, rest, reward, rooting out,
   sentence of death, shades of death, shadow of death, silencing,
   sleep, smotheration, smothering, snuffing, snuffing out,
   somatic death, stifling, strangulation, suffocation,
   summons of death, suppression, uprooting, vanishing,
   vanishing point, voiding, wipe



The Devil's Dictionary (1881-1906):

EXTINCTION, n.  The raw material out of which theology created the
future state.