A mountain bike is a type of bicycle that is suitable for riding over rough ground. It has a strong frame and thick tyres.
mountain bike in British English
noun
a type of sturdy bicycle with at least 16 and up to 21 gears, straight handlebars, and heavy-duty tyres
mountain bike in American English
a bicycle having a sturdy frame, handlebars straight across, wide 26-inch tires with a knobby tread, as many as 21-24 gears, etc., that is suitable for both road and off-road use