Pet overpopulation was originally used to describe a situation in which the volume of kittens and puppies overwhelmed the capacity of pet owners and shelters to care for them all and the excess were humanely destroyed in shelters. Over time, as puppies and kittens came to make up an increasingly smaller share of shelter admissions, some continued to refer to the overpopulation of animal shelters as pet overpopulation even though many of the impounded animals were unsocialized cats that had never been kept as pets and the reasons for many impoundments arose from a characteristic of the animal or pet owner, not the overall size of the pet population. Although there continued to be overpopulation, it was in the sheltering system, not in the total pet population. For this reason, the term shelter overpopulation will be used to describe a situation in which the total number of animals impounded from all sources substantially exceeds the carrying capacity
of shelters in a community and the rate at which sheltered animals can be placed in appropriate adoptive homes.
Euthanasia in its common usage refers to taking steps to end the life of a person suffering from a terminal illness or incurable condition. With reference to animal shelters, the term has been applied both to taking steps to end the life of an animal that is severely injured or dying and to taking the life of an animal that is healthy or suffers from a treatable condition in order to control the size of the shelter population. To differentiate between these two contexts, medical euthanasia will be used to refer to the former situation and population control euthanasia to the latter. This will allow the population control euthanasia rate to serve as a measure of shelter overpopulation. Shelter overpopulation will have been eliminated when the only euthanasias performed in shelters are medical euthanasias that have not been induced by conditions at the