Rattlesnake n. : Any one of several species of venomous American snakes belonging to the genera Crotalus and Caudisona, or Sistrurus. 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 diamond rattlesnake of the South (C. adamanteus), are the best known. See Illust. of Fang.

Example:
How long does a Rattlesnake spasm when you cut its head off?

Taxonomy
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Defination from the web:
rattlesnake is not cornered or imminently threatened, it will usually attempt to flee from encounters with humans, but will not always do so. ... In the U.S. the Mojave Rattlesnake (Crotalus scutulatus) in Arizona and parts of California has a neurotoxic venom component known ...