The Collaborative International Dictionary of English v.0.48:

Squirrel \Squir"rel\ (skw[~e]r"r[e^]l or skw[i^]r"-; 277), n.
   [OE. squirel, OF. esquirel, escurel, F. ['e]cureuil, LL.
   squirelus, squirolus, scuriolus, dim. of L. sciurus, Gr.
   si`oyros; skia` shade + o'yra` tail. Cf. Shine, v. i.]
   1. (Zool.) Any one of numerous species of small rodents
      belonging to the genus Sciurus and several allied genera
      of the family Sciuridae. Squirrels generally have a
      bushy tail, large erect ears, and strong hind legs. They
      are commonly arboreal in their habits, but many species
      live in burrows.
      [1913 Webster]

   Note: Among the common North American squirrels are the gray
         squirrel (Sciurus Carolinensis) and its black
         variety; the fox, or cat, squirrel (Sciurus cinereus,
         or Sciurus niger) which is a large species, and
         variable in color, the southern variety being
         frequently black, while the northern and western
         varieties are usually gray or rusty brown; the red
         squirrel (see Chickaree); the striped, or chipping,
         squirrel (see Chipmunk); and the California gray
         squirrel (Sciurus fossor). Several other species
         inhabit Mexico and Central America. The common European
         species (Sciurus vulgaris) has a long tuft of hair on
         each ear. The so-called Australian squirrels are
         marsupials. See Petaurist, and Phalanger.
         [1913 Webster]

   2. One of the small rollers of a carding machine which work
      with the large cylinder.
      [1913 Webster]

   Barking squirrel (Zool.), the prairie dog.

   Federation squirrel (Zool.), the striped gopher. See
      Gopher, 2.

   Flying squirrel (Zool.). See Flying squirrel, in the
      Vocabulary.

   Java squirrel. (Zool.). See Jelerang.

   Squirrel corn (Bot.), a North American herb (Dicentra
      Canadensis) bearing little yellow tubers.

   Squirrel cup (Bot.), the blossom of the Hepatica triloba,
      a low perennial herb with cup-shaped flowers varying from
      purplish blue to pink or even white. It is one of the
      earliest flowers of spring.

   Squirrel fish. (Zool.)
      (a) A sea bass (Serranus fascicularis) of the Southern
          United States.
      (b) The sailor's choice (Diplodus rhomboides).
      (c) The redmouth, or grunt.
      (d) A market fish of Bermuda (Holocentrum Ascensione).


   Squirrel grass (Bot.), a pestiferous grass (Hordeum
      murinum) related to barley. In California the stiffly
      awned spikelets work into the wool of sheep, and into the
      throat, flesh, and eyes of animals, sometimes even
      producing death.

   Squirrel hake (Zool.), a common American hake (Phycis
      tenuis); -- called also white hake.

   Squirrel hawk (Zool.), any rough-legged hawk; especially,
      the California species Archibuteo ferrugineus.

   Squirrel monkey. (Zool.)
      (a) Any one of several species of small, soft-haired South
          American monkeys of the genus Callithrix. They are
          noted for their graceful form and agility. See
          Teetee.
      (b) A marmoset.

   Squirrel petaurus (Zool.), a flying phalanger of Australia.
      See Phalanger, Petaurist, and Flying phalanger under
      Flying.

   Squirrel shrew (Zool.), any one of several species of East
      Indian and Asiatic insectivores of the genus Tupaia.
      They are allied to the shrews, but have a bushy tail, like
      that of a squirrel.

   Squirrel-tail grass (Bot.), a grass (Hordeum jubatum)
      found in salt marshes and along the Great Lakes, having a
      dense spike beset with long awns.
      [1913 Webster]

WordNet (r) 3.0 (2006):

squirrel
    n 1: a kind of arboreal rodent having a long bushy tail
    2: the fur of a squirrel