36 CFR 78.2: Definitions.

Federal Agency Head means the highest administrative official of a Federal agency, or designee.

Imminent Threat to the National Security means the imminence of any natural, technological, or other occurrence which, in determination of a Federal Agency Head, because of its size or intent, seriously degrades or threatens the national security of the United States such that an emergency action would be impeded if the Federal Agency were to concurrently meet its historic preservation responsibilities under section 110 of the National Historic Preservation Act, as amended.

Major Natural Disaster means any hurricane, tornado, storm, flood, high water, tidal wave, earthquake, volcanic eruption, landslide, snowstorm, fire, explosion, or other catastrophe, in any part of the United States which, in the determination of a Federal Agency Head, causes damage of sufficient severity and magnitude such that an emergency action is necessary to the preservation of human life or property, and that such emergency action would be impeded if the Federal Agency were to concurrently meet its historic preservation responsibilities under section 110 of the National Historic Preservation Act, as amended.