From AMS Glossary
Ice caps are considerably smaller than ice sheets, and it is felt improper to refer to the Greenland and Antarctic Ice Sheets as ice caps. Most ice caps are most probably remnants of the Quaternary Ice Age. The term was first used for the supposedly perennial ice cover at both poles of the earth. However, since it has been found that the ice of arctic waters is largely seasonal, the use of this term to denote arctic polar ice is now considered improper.