mountain bike

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

• noun A sturdy bicycle that is designed for use on hills and rough surfaces and is usually equipped with wide, knobby tires and shock absorbers.

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

• noun A bicycle specially designed for off-road riding, typically with straight, flat handlebars, a heavier frame than road bikes, higher ground clearance, and wider tires, sometimes with knobs on them for greater traction.

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

• noun a bicycle with a sturdy frame and fat tires; originally designed for riding in mountainous country