Wikipedia suggests that "An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets." The main idea being that complex overlapping cycles occasionally, and very slowly, change global temperature levels so much that although the North Pole and South Pole are fixed in place and never change, the portion of the Earth that experiences polar weather (ice sheets and glaciers) increases and decreases over long periods of time.