A glacier was a large slow-moving mass of ice.
The Twelfth Doctor surmised that the Ice Age on Earth was started by glaciers "pop[ping] out of nowhere" as he explained what he called the "Tree Age". (TV: In the Forest of the Night)
To stop an advancing glacier in 5000 AD, Brittanicus Base was established by Great Britain. While using an ioniser to melt the ice, they uncovered a troop of Ice Warriors which had been frozen for thousands of years when their spaceship had crashed in the area. (TV: The Ice Warriors)
The Northern Glacier on the Ood Sphere was where Ood Operations found the Ood Brain. (TV: Planet of the Ood)
The Flood was sealed in a glacier on Mars by the Ice Warriors in the 9th century. It was accidentally freed by Bowie Base One in 2059. (COMIC: The Wolves of Winter, TV: The Waters of Mars)
On Bruhl, the First Doctor observed a glacier which he estimated to be at least twenty miles long. (PROSE: Justice of the Glacians)