A lahar refers to a mixture of rock, mud, and water that flows down from a volcano, typically along a river valley. The term originated in Indonesia.
Lahars have the consistency of concrete: wet when moving, then solid when stopped. Lahars can range in size: the lahar produced by Mount Rainier in the USA 5600 years ago produced a wall of mud 600 feet deep in the White River canyon.
Lahars can be extremely dangerous, because of their energy and speed: large lahars can flow several tens of meters per second. Lahars can bury and entomb large areas. The lahars from the Nevado del Ruiz eruption in Colombia in 1985 killed an estimated 23,000 people.
Lahars have several possible causes 
- Snow and glaciers can be melted by a pyroclastic flow during an eruption
- A flood caused by a glacier, lake breakout, or heavy rainfall can release a lahar.
- A sudden landslide, not associated with a major eruption, can trigger a lahar.
Several mountains in the world, including Mount Rainier, Mount Ruapehu in New Zealand, and Galunggung in Indonesia are considered dangerous due to a risk of lahars. Several towns near Mount Rainier, including Orting, Washington, are built on top of lahar deposits that are only ~500 years old. Lahars are predicted to reach Orting every 500-1000 years, so the town has considerable risk. The USGS has set up lahar warning sirens in Orting, so that people can flee the approaching debris flow. A lahar warning system is also being set up at Mount Ruapehu by the New Zealand Department of Conservation.