Natural event
Natural event means an event and its resulting emissions, which may recur at the same location, in which human activity plays little or no direct causal role. For purposes of the definition of a natural event, anthropogenic sources that are reasonably controlled shall be considered to not play a direct role in causing emissions.
Source
40 CFR § 50.1
Scoping language
As used in this part, all terms not defined herein shall have the meaning given them by the Act.