Real Dictionary

mountain
countable noun: A mountain is a very high area of land with steep sides.
quantifier: If you talk about a mountain of something, or mountains of something, you are emphasizing that there is a large amount of it.

mountain in American English
a natural raised part of the earth's surface, usually rising more or less abruptly, and larger than a hill
of a mountain or mountains
noun: a natural elevation of the earth's surface rising more or less abruptly to a summit, and attaining an altitude greater than that of a hill, usually greater than 2000 ft. (610 m)

mountain in British English
noun
noun: a natural upward projection of the earth's surface, higher and steeper than a hill and often having a rocky summit