The Collaborative International Dictionary of English v.0.48:

Euthanasia \Eu`tha*na"si*a\n. [NL., fr. Gr. ?; e'y^ well + ?
   death, ?, ?, to die: cf. F. euthanasie.]
   1. An easy death; a mode of dying to be desired. "An
      euthanasia of all thought." --Hazlitt.
      [1913 Webster]

            The kindest wish of my friends is euthanasia.
                                                  --Arbuthnot.

   2. the act or process of putting to death for humane
      purposes; -- used to refer to the killing of animals in
      order to relieve or avoid pain.
      [PJC]

WordNet (r) 3.0 (2006):

euthanasia
    n 1: the act of killing someone painlessly (especially someone
         suffering from an incurable illness) [syn: euthanasia,
         mercy killing]