14 CFR 235.1: Definitions.

For the purposes of this part:

Air transport includes the entire period during which an animal is in the custody of an air carrier, from the time that the animal is tendered to the air carrier prior to departure until the air carrier tenders the animal to the owner, guardian or representative of the shipper of the animal at the animal's final destination. It does not include animals that accompany a passenger at his or her seat in the cabin and of which the air carrier does not take custody.

Animal means any warm- or cold-blooded animal which, at the time of transportation, is being kept as a pet in a family household in the United States and any dog or cat which, at the time of transportation, is shipped as part of a commercial shipment on a scheduled passenger flight, including shipments by trainers and breeders.