Source : Webster's Revised Unabridged Dictionary (1913)

Water snake \Wa"ter snake`\ (Zo["o]l.)
      (a) A common North American colubrine snake ({Tropidonotus
          sipedon}) which lives chiefly in the water.
      (b) Any species of snakes of the family {Homalopsid[ae]},
          all of which are aquatic in their habits.

Snake \Snake\, n. [AS. snaca; akin to LG. snake, schnake, Icel.
   sn[=a]kr, sn?kr, Dan. snog, Sw. snok; of uncertain origin.]
   (Zo["o]l.)
   Any species of the order Ophidia; an ophidian; a serpent,
   whether harmless or venomous. See {Ophidia}, and {Serpent}.

   Note: Snakes are abundant in all warm countries, and much the
         larger number are harmless to man.

   {Blind snake}, {Garter snake}, {Green snake}, {King snake},
   {Milk snake}, {Rock snake}, {Water snake}, etc. See under
      {Blind}, {Garter}, etc.

   {Fetich snake} (Zo["o]l.), a large African snake ({Python
      Seb[ae]}) used by the natives as a fetich.

   {Ringed snake} (Zo["o]l.), a common European columbrine snake
      ({Tropidonotus natrix}).

   {Snake eater}. (Zo["o]l.)
   (a) The markhoor.
   (b) The secretary bird.

   {Snake fence}, a worm fence (which see). [U.S.]

   {Snake fly} (Zo["o]l.), any one of several species of
      neuropterous insects of the genus {Rhaphidia}; -- so
      called because of their large head and elongated neck and
      prothorax.

   {Snake gourd} (Bot.), a cucurbitaceous plant ({Trichosanthes
      anguina}) having the fruit shorter and less snakelike than
      that of the serpent cucumber.

   {Snake killer}. (Zo["o]l.)
   (a) The secretary bird.
   (b) The chaparral cock.

   {Snake moss} (Bot.), the common club moss ({Lycopodium
      clavatum}). See {Lycopodium}.

   {Snake nut} (Bot.), the fruit of a sapindaceous tree
      ({Ophiocaryon paradoxum}) of Guiana, the embryo of which
      resembles a snake coiled up.

   {Tree snake} (Zo["o]l.), any one of numerous species of
      colubrine snakes which habitually live in trees,
      especially those of the genus {Dendrophis} and allied
      genera.

Source : WordNet®

water snake
     n : any of various mostly harmless snakes that live in or near
         water