service dog

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A dog that has been specially trained to assist a disabled person with certain daily tasks, such as picking up an object from the floor.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A canine, such as a seeing-eye dog or a hearing-ear dog, which assists a person who is physically handicapped or who suffers from a mental disability such as post-traumatic stress disorder.
• noun A canine which assists the work of military or law enforcement officials.