An unplanned ignition caused by lightning, volcanoes, unauthorized, and accidental human-caused actions and escaped prescribed fires.
Source: National Wildfire Coordinating Group. " Terminology Updates Resulting from Release of the Guidance for the Implementation of Federal Wildland Fire Management Policy (2009) Attachment A: Terms whose definition and intent has changed with the Guidance for the Implementation of Federal Wildland Fire Management Policy (2009). Accessed 7 July, 2011. http://www.nwcg.gov/