From AMS Glossary
Snow that has been compressed at low temperatures and that sets into a tough substance of considerably greater strength than uncompressed snow (from Glossary of Arctic and Subarctic Terms 1955).
Arctic, Desert, Tropic Information Center (ADTIC) Research Studies Institute 1955. Glossary of Arctic and Subarctic Terms. ADTIC Pub. A-105, Maxwell AFB, AL, . 90 pp.