Definition of mountain noun from the Oxford Advanced Learner's Dictionary

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mountain

noun
ˈmaʊntən
 
; ˈmaʊntn
 
 
1 a very high hill, often with rocks near the topa chain/range of mountainsto climb a mountainWe spent a week walking in the mountains.to enjoy the mountain air/scenerymountain roads/streams/villagesa mountain rescue teamThere is still snow on the mountain tops. 2 mountain of something (informal) a very large amount or number of somethinga mountain of workWe made mountains of sandwiches.the problem of Europe's butter mountain (= the large amount of butter that has to be stored because it is not needed)
Idioms

make a mountain out of a molehill

(disapproving) to make an unimportant matter seem important