seat belt

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

• noun A safety strap or harness designed to hold a person securely in a seat, as in a motor vehicle or aircraft.

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

• noun A restraining belt attached to a vehicle seat which is fastened around passengers and which is intended to keep an occupant in place so as to avoid injuries in accidents where the occupant could be thrown against a solid object.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a safety belt used in a car or plane to hold you in your seat in case of an accident