A lahar is a type of volcanic mudflow or debris flow that occurs when volcanic ash, pyroclastic material, and water mix, typically due to heavy rainfall, the melting of ice or snow, or the disruption of a crater lake. Lahars can be highly destructive, flowing rapidly down valleys and causing significant damage to communities and infrastructure.
Reference: Vallance, J. W. (2000). “Lahars.” In: Sigurdsson, H. (Ed.), Encyclopedia of Volcanoes. Academic Press, pp. 601-616.