type of Special Use Airspace "over international waters in which, because of military exercise, non-participating aircraft may be at risk."
Jane's Aerospace Dictionary, 1988, 3rd Ed.
U.S.: A warning area is airspace of defined dimensions, extending from 3 NM outward from the coast of the United States, that contains activity that may be hazardous to nonparticipating aircraft. The purpose of such warning areas is to warn nonparticipating pilots of the potential danger. A warning area may be located over domestic or international waters or both. (source:FAA, Pilot/Controller Glossary)