The Collaborative International Dictionary of English v.0.48:

Rattlesnake \Rat"tle*snake`\ (r[a^]t"t'l*sn[=a]k`), n. (Zool.)
   Any one of several species of venomous American snakes
   belonging to the genera Crotalus and Caudisona, or
   Sistrurus; sometimes also called rattler. They have a
   series of horny interlocking joints at the end of the tail
   which make a sharp rattling sound when shaken. The common
   rattlesnake of the Northern United States (Crotalus
   horridus), and the diamondback rattlesnake (also called
   diamondback rattler, and diamondback) of the South and
   East (Crotalus adamanteus) and West (Crotalus atrox), are
   the best known. See Illust. of Fang.
   [1913 Webster +PJC]

   Ground rattlesnake (Zool.), a small rattlesnake (Caudisona
      miliaria or Sistrurus miliaria) of the Southern United
      States, having a small rattle. It has nine large scales on
      its head.

   Rattlesnake fern (Bot.), a common American fern
      (Botrychium Virginianum) having a triangular decompound
      frond and a long-stalked panicle of spore cases rising
      from the middle of the frond.

   Rattlesnake grass (Bot.), a handsome American grass
      (Glyceria Canadensis) with an ample panicle of rather
      large ovate spikelets, each one composed of imbricated
      parts and slightly resembling the rattle of the
      rattlesnake. Sometimes called quaking grass.

   Rattlesnake plantain (Bot.), See under Plantain.

   Rattlesnake root (Bot.), a name given to certain American
      species of the composite genus Prenanthes (Prenanthes
      alba and Prenanthes serpentaria), formerly asserted to
      cure the bite of the rattlesnake. Called also lion's
      foot, gall of the earth, and white lettuce.

   Rattlesnake's master (Bot.)
   (a) A species of Agave (Agave Virginica) growing in the
       Southern United States.
   (b) An umbelliferous plant (Eryngium yuccaefolium) with
       large bristly-fringed linear leaves.
   (c) A composite plant, the blazing star (Liatris
       squarrosa).

   Rattlesnake weed (Bot.), a plant of the composite genus
      Hieracium (Hieracium venosum); -- probably so named
      from its spotted leaves. See also Snakeroot.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Crotalus atrox
    n 1: largest and most dangerous North American snake; of
         southwestern United States and Mexico [syn: Western
         diamondback, Western diamondback rattlesnake, Crotalus
         atrox]