The Collaborative International Dictionary of English v.0.48:

passenger pigeon \passenger pigeon\ (Zool.),
   A once common wild pigeon of North America (Ectopistes
   migratorius), now extinct. It was so called on account of
   its extensive migrations.
   [1913 Webster +PJC]

   Note: The passenger pigeon presents a striking example of how
         dramatic a negative influence man can have on other
         species. The population of the passenger pigeon is
         estimated to have been at one time as high as five
         billion in North America, but over a period of about
         one hundred years large scale hunting for use as food
         and killing for "sport" reduced the numbers below that
         necessary to sustain existence of the species. At one
         time over 200,000 birds were shipped to the food
         markets in one day. The last wild pigeon is believed to
         have died in 1900, and the last bird in captivity died
         in 1914. It is a rare example of the date of a man-made
         extinction being recorded with certainty.
         [PJC] passe-partout

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):

Ectopistes migratorius
    n 1: gregarious North American migratory pigeon now extinct
         [syn: passenger pigeon, Ectopistes migratorius]