a mountain: a high rise of land, taller than a hill, whether a particular peak or the open hill country
This is the ordinary Greek word for a mountain, an elevation of land that stands up taller than the gentler rises around it. In the Gospels it is most often a definite place: a peak someone climbs to pray, to teach, or to meet God, and several of those mountains carry deep memory in the Bible, such as Sinai where the law was given and the high hill near Jerusalem where Jesus prayed.
The same word also stretches to mountains taken together as rough, remote country: the wild slopes where a lost sheep strays, the high ground people flee to when danger comes, or the lonely places where the persecuted and the tormented end up. Whether it points to one peak or to the hills as a whole depends entirely on the setting, not on a change in what the word itself means.
Because mountains are the most solid, unmovable things people knew, the word naturally lends itself to vivid pictures. A faith that could tell a mountain to get up and throw itself into the sea is a faith that moves the immovable, and when the prophets and the book of Revelation describe mountains being leveled, shaken, or fleeing away, they are using the very thing that seems most permanent to show how the whole created order answers to God.