From Longman Dictionary of Contemporary Englishmountain bikeˈmountain ˌbike noun [countable] 
TTBa strong bicycle with a lot of gears and wide tyres, specially designed for riding up hills and on rough ground
TTBa strong bicycle with a lot of gears and wide tyres, specially designed for riding up hills and on rough groundExamples from the Corpus
mountain bike• A man completed all the Munros with a mountain bike.• A colourful explanatory leaflet is available from all national park outlets and most responsible mountain bike shops in the area.• Finally, the voluntary mountain bike restriction for Snowdon mentioned a couple of issues ago has been agreed between the interested parties. 