A lahar is a mudslide made of pyroclastic material and water that comes down from a volcano, usually by a river valley. The word 'lahar' came from the Javanese language of Indonesia.
Lahars are as thick as concrete and are watery when moving, then hard when not. Lahars can be huge. The Osceola lahars made 5,600 years ago by Mount Rainier in Washington produced a wall of mud 460 feet deep in the White River canyon and extends over an area of over 330 square 130 sq miles.
Lahars can be very dangerous, because of their power and speed. Big lahars can flow yards per second and can flow for many miles, causing great destruction wherever they go. A lahar from the Nevado del Ruiz volcano in Colombia in 1985 made the Armero tragedy, which killed 23,000 people when Armero was, buried under 16 feet of mud.
Lahars have many possible reasons.
Snow and glaciers can be melted by lava flow during an eruption
A flood caused by a glacier, lake breakout, or heavy rainfall can make a lahar, also called glacier run.
Lahars are usually associated with the effects of volcanoes, lahars can happen without volcanic activity, as long as the pieces all fit together to cause the collapse of mud from existing volcanic ash deposits.
A few mountains in the world, Mount Rainier in the USA, Mount Ruapehu in New Zealand, and Galunggung in Indonesia, are dangerous due to the risk of lahars. Many towns in the Puyallup River valley in Washington are built on top of lahars that are about 500 years old. Lahars are thought to flow through the valley every 500-1,000 years. This is bad news for those people living there.
So in conclusion, a lahar is a giant mudflow that can kill tons of people unless they are evacuated.