Euthanasia is the action of directly causing the quick and painless death of a person, or omitting to prevent it when intervention was within the agent’s powers. It is usually understood that euthanasia is performed only with the intention of relieving suffering, and where death is perceived as the greater good or lesser evil for the patient. Active euthanasia means acting to bring the death about, passive euthanasia means not preventing it (see acts/omissions doctrine). Although medical codes more commonly allow doctors to follow the passive option than they allow them to actively intervene to cause death, there are problems of drawing the distinction in particular cases. There is also the unpleasant possibility that the passive withholding of treatment may condemn the patient to a lingering and painful death, which an active intervention would have prevented. Euthanasia is something many people would wish for themselves if life were to become unbearable, although it is frequently opposed. One set of worries concerns its expansion to form a climate in which old people are encouraged to go quietly, and to feel guilty about hanging on. It is also opposed on the curious grounds of respect for life. In many systems of law it would be illegal to give a suffering person a painless death when, if a cat or dog were in the same condition, it would be illegal not to do so.

http://www.bbc.co.uk/religion/ethics/euthanasia/ A survey of related religious, medical, and legal debates