The Collaborative International Dictionary of English v.0.48:

Penguin \Pen"guin\ (p[e^]n"gw[i^]n), n. [Perh. orig. the name of
   another bird, and fr. W. pen head + gwyn white; or perh. from
   a native South American name.]
   1. (Zool.) Any bird of the order Impennes, or Ptilopteri.
      They are covered with short, thick feathers, almost
      scalelike on the wings, which are without true quills.
      They are unable to fly, but use their wings to aid in
      diving, in which they are very expert. See King penguin,
      under Jackass.
      [1913 Webster]

   Note: Penguins are found in the south temperate and antarctic
         regions. The king penguins (Aptenodytes Patachonica,
         and Aptenodytes longirostris) are the largest; the
         jackass penguins (Spheniscus) and the rock hoppers
         (Catarractes) congregate in large numbers at their
         breeding grounds.
         [1913 Webster]

   2. (Bot.) The egg-shaped fleshy fruit of a West Indian plant
      (Bromelia Pinguin) of the Pineapple family; also, the
      plant itself, which has rigid, pointed, and spiny-toothed
      leaves, and is used for hedges. [Written also pinguin.]
      [1913 Webster]

   Arctic penguin (Zool.), the great auk. See Auk.
      [1913 Webster]

WordNet (r) 3.0 (2006):

penguin
    n 1: short-legged flightless birds of cold southern especially
         Antarctic regions having webbed feet and wings modified as
         flippers