Euthanasia is the act of causing a person's or animal's death, without inflicting pain, to end suffering, like when a veterinarian performs euthanasia on a dog that is in great pain and has no chance of recovery.
To correctly pronounce euthanasia, remember that it sounds like "youth in Asia." Euthanasia is sometimes referred to as mercy killing, meant to spare a living thing a slow, painful death. Originally a Greek word, euthanasia means "an easy or happy death," as eu- means "good" and thanatos means "death." The use of the word as "legally sanctioned mercy killing" is first recorded in English in 1869.