Euthanasia (from the Greek εὐθανασία meaning "good death": εὖ, eu (well or good) + θάνατος, thanatos (death)) refers to the practice of ending a life in a manner which relieves pain and suffering. According to the House of Lords Select Committee on Medical Ethics, the precise definition of euthanasia is "a deliberate intervention undertaken with the express intention of ending a life, to relieve intractable suffering."

The judicial sense of the term "homicide" includes any intervention undertaken with the express intention of ending a life, even to relieve intractable suffering. Thus judicially, a "mercy killing" or euthanasia is generally considered to be a criminal homicide and is normally used as a synonym of homicide committed at a request made by the patient

The controversy surrounding euthanasia centers around a two-pronged argument by opponents which characterises euthanasia as either voluntary "suicides", or as involuntary murders. (Hence, opponents argue that a broad policy of "euthanasia" is tantamount to eugenics). Much hinges on whether a particular death was considered an "easy," "painless," or "happy" one, or whether it was a "wrongful death". Proponents typically consider a death that increased suffering to be "wrongful," while opponents typically consider any deliberate death as "wrongful." "Euthanasia's" original meaning introduced the idea of a "rightful death" beyond that only found in natural deaths.

Euthanasia is the most active area of research in contemporary bioethics.