Definition of service dogs:
(n) :
(chiefly Canada, US) A dog which assists a person who has a physical disability or a mental disability such as post-traumatic stress disorder.
(n) :
A dog which serves people by helping them with activities of daily living, with search and rescue, or with other (non-draft and nonfarming) tasks.
(n) :
(law enforcement, military) A dog which assists the work of military or law enforcement officers.