A glacial age, also known as an ice age, was a period when a planet experienced a long-term temperature reduction, with a large part of its surface covered by ice sheets and glaciers.
A glacial age almost wiped out the population of Sigma Draconis VI around 50,000 years ago, leaving only the tropical zone ice-free. It still affected the planet by 2268. (TOS: "Spock's Brain")
In 2366, Garin feared that the impact of a moon on Bre'el IV would lead to the formation of a dust cloud around the planet, resulting in a significant temperature reduction and thus an ice age. (TNG: "Deja Q")
The Sarpeidon Ice Age and Therinian Ice Age were other examples of such a period. (TOS: "All Our Yesterdays"; VOY: "The Q and the Grey")