Real Dictionary

glacier
countable noun: A glacier is an extremely large mass of ice which moves very slowly, often down a mountain valley.

glacier in American English
a large mass of ice and snow that forms in areas where the rate of snowfall constantly exceeds the rate at which the snow melts: it moves slowly outward from the center of accumulation or down a mountain until it melts or breaks away
noun: an extended mass of ice formed from snow falling and accumulating over the years and moving very slowly, either descending from high mountains, as in valley glaciers, or moving outward from centers of accumulation, as in continental glaciers

glacier in British English
noun: a slowly moving mass of ice originating from an accumulation of snow. It can either spread out from a central mass ( continental glacier) or descend from a high valley ( alpine glacier)