A lahar /ˈlɑːhɑr/ is a type of mudflow or debris flow composed of a slurry of pyroclastic material, rocky debris, and water. The material flows down from a volcano, typically along a river valley. The term is a shortened version of "berlahar" which originated in the Javanese language of Indonesia.^{[citation needed]}

"Lahar" is an Indonesian word that describes volcanic mudflows or debris flows. Lahars have the consistency, viscosity and approximate density of concrete: fluid when moving, solid at rest. Lahars can be huge. The Osceola lahar produced by Mount Rainier (Washington) some 5,600 years ago resulted in a wall of mud 140 metres (460 ft) deep in the White River canyon, which covered an area of over 330 square kilometres (130 sq mi), for a total volume of 2.3 cubic kilometres (0.55 cu mi). A lahar of sufficient size and intensity can raze virtually any structure in its path, and is capable of carving its own pathway, making the prediction of its course difficult. Conversely, a Lahar quickly loses force when it leaves the channel of its flow: even frail huts may remain standing, while at the same time being buried to the roof line in mud. A lahar's viscosity decreases with time, and can be further thinned by rain, but it nevertheless solidifies quickly when coming to a stop.