An ice cap is a thick layer of ice and snow that permanently covers a large area of land, usually in polar or high-altitude regions. Ice caps are smaller than ice sheets (like the enormous ones covering Antarctica and Greenland) but still massive, sometimes covering thousands of square miles.

Ice caps form over thousands of years as snow falls, compresses, and turns into dense ice. They can be hundreds or even thousands of feet thick. Unlike glaciers that flow down valleys, ice caps spread outward in all directions from their highest point, like frosting spreading across the top of a cake.

Scientists study ice caps carefully because they contain layers of ice that preserve ancient air bubbles and particles, creating a record of Earth's climate going back thousands of years. Ice caps also affect sea levels: when they melt, that water flows into the ocean. In recent decades, many ice caps have been shrinking due to warming temperatures, which concerns scientists who monitor changes in Earth's climate.

The term can also refer to the polar ice caps at Earth's North and South Poles, or even ice caps discovered on other planets like Mars, where spacecraft have photographed bright white regions of frozen water ice and carbon dioxide ice.