The Collaborative International Dictionary of English v.0.48:

autotomy \autotomy\ n.
   1. spontaneous removal or casting off of a body part (as the
      tail of a lizard or claw or a lobster) especially when the
      organism is injured or under attack.
      [WordNet 1.5]

WordNet (r) 3.0 (2006):

autotomy
    n 1: spontaneous removal or casting off of a body part (as the
         tail of a lizard or claw of a lobster) especially when the
         organism is injured or under attack