The Codes of Federal Regulation for the Americans with Disabilities Act of 1990 state,
"Service animal means any guide dog, signal dog, or other animal individually trained to do work or perform tasks for the benefit of an individual with a disability, including, but not limited to, guiding individuals with impaired vision, alerting individuals with impaired hearing to intruders or sounds, providing minimal protection or rescue work, pulling awheelchair, or fetching dropped items."
This definition is currently under review. The U.S. Department of Justice issued a "Notice of Proposed Rulemaking" indicating the Department's intent to modify this definition to read:
"Service animal means any dog or other common domestic animal individually trained to do work or perform tasks for the benefit of an individual with a disability, including, but not limited to, guiding individuals who are blind or have low vision, alerting individuals who are deaf or hard of hearing to the presence of people or sounds, providing minimal protection or rescue work, pulling a wheelchair, fetching items, assisting an individual during a seizure, retrieving medicine or the telephone, providing physical support and assistance with balance and stability to individuals with mobility disabilities, and assisting individuals, including those with cognitive disabilities, with navigation. The term service animal includes individually trained animals that do work or perform tasks for the benefit of individuals with disabilities, including psychiatric, cognitive, and mental disabilities. The term service animal does not include wild animals (including nonhuman primates born in captivity), reptiles, rabbits, farm animals (including any breed of horse, miniature horse, pony, pig, or goat), ferrets, amphibians, and rodents. Animals whose sole function is to provide emotional support, comfort, therapy, companionship, therapeutic benefits, or to promote emotional well-being are not service animals."
As of July 2009, this change has not been made officially and has not gone into effect.
Some other laws, especially some state laws, may use a different legal definition of "service animal." Consult a qualified attorney to learn which law and which definition apply in your specific situation.