a slowly moving mass of ice
"The massive glacier carved deep valleys through the mountains over thousands of years."
A large body of ice which flows under its own mass, usually downhill.
"A massive glacier slowly crept down the mountain valley, carving deep channels into the rock beneath it."
In plain English: A glacier is a huge, slow-moving river of ice that forms on land when snow piles up over many years and turns into solid ice.
"The massive glacier slowly carved its way through the mountain valley over thousands of years."
Usage: A glacier refers to the massive river-like flow of ice that moves slowly over land due to gravity and pressure. Use this term specifically when describing these natural formations rather than smaller piles of snow or stationary ice blocks.
The word "glacier" entered English in 1744 via French and Franco-Provençal, where it originally referred to ice or a place for making ice before coming to mean a large river of frozen water moving slowly over land. Its ultimate roots lie in the Latin glaciēs, meaning "ice," though the origin of that specific root remains uncertain.