Animal euthanasia is the act of putting to death painlessly or allowing to die, as by withholding extreme medical measures, an animal suffering from an incurable, especially a painful, disease or condition. Euthanasia

Euthanasia

Euthanasia refers to the practice of intentionally ending a life in order to relieve pain and suffering....

 methods are designed to cause minimal pain and distress. Euthanasia is distinct from animal slaughter

Animal slaughter

Slaughter is the term used to describe the killing and butchering of animals, usually for food. Commonly it refers to killing and butchering of domestic livestock ....

 and pest control

Pest control

Pest control refers to the regulation or management of a species defined as a pest, usually because it is perceived to be detrimental to a person's health, the ecology or the economy.-History:...

, which are performed for purposes other than an act of mercy, although in some cases the killing procedure is the same.

In domesticated animals, this process is commonly referred to by euphemism

Euphemism

A euphemism is the substitution of a mild, inoffensive, relatively uncontroversial phrase for another more frank expression that might offend or otherwise suggest something unpleasant to the audience...

s such as "lay down", "put down", "put to sleep", or "put out of its/his/her misery".
Pets are almost always euthanized by intravenous injection, typically a very high dose of pentobarbital

Pentobarbital

Pentobarbital is a short-acting barbiturate that was first synthesized in 1928. Pentobarbital is available as both a free acid and a sodium salt, the former of which is only slightly soluble in water and ethanol....

 or sodium thiopental

Sodium thiopental

Sodium thiopental, better known as Sodium Pentothal , thiopental, thiopentone sodium, or Trapanal , is a rapid-onset short-acting barbiturate general anaesthetic...

.