As defined by Department of Housing and Urban Development:

“An assistance animal that works, provides assistance, or performs tasks for the benefit of a person with a disability, or provides emotional support that alleviates one or more symptoms or effects of a person’s disability.”

As defined by Americans with Disabilities Act:

“Any dog that is individually trained to do work or perform tasks for the benefit of an individual with a disability, including a physical, sensory, psychiatric, intellectual, or other mental disability." Miniature horses have been added as a specific provision to the ADA.

As defined by Fair Housing Act and Air Carrier Access Act:

An emotional support animal may be an animal of any species, the use of which is supported by a qualified physician, psychiatrist, or other mental health professional on the basis of a disability-related need. Emotional support animals may be permitted as reasonable accommodations for persons with disabilities under the Fair Housing Act. The Air Carrier Access Act provides specific allowances for emotional support animals traveling on airlines, though documentation may need to be provided.

As defined by AVMA policy "Animal-Assisted Interventions: Definitions":

A therapy animal participates in animal-assisted therapy or an "intervention in which an animal meeting specific criteria is an integral part of the treatment process. ...Animal-assisted therapy is provided in a variety of settings, and may be group or individual in nature."