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]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "euthanasia":
   bane, blood, bloodletting, bloodshed, braining, dealing death,
   destruction, destruction of life, dispatch, execution,
   extermination, flow of blood, gore, immolation, kill, killing,
   lapidation, martyrdom, martyrization, mercy killing, natural death,
   poisoning, quiet end, ritual killing, ritual murder, sacrifice,
   shooting, slaughter, slaying, stoning, taking of life,
   welcome release