An avalanche is a big pile of snow, dirt, and other objects, that come crashing down a mountainside. When there is an avalanche it is usually a sliding avalanche. A sliding avalanche is when the snow piles up and weighs too much to stay on top of the slope and slides down the mountain.
Here is a picture of an avalanche. You can see the dirt and rocks that came down the mountain with the snow.
There are two other types of avalanches also. They are called wind avalanches, and summer avalanches. A wind avalanche happens when a strong wind blows dry snow down the side of the mountain. A summer avalanche happens in the summer when a glacier starts to melt. A big piece of ice is hanging off the glacier attached by a small piece of ice. When the small piece melts, the large piece rolls down the mountain.
Avalanches are most likely to happen when loose layers of snow fall on top of ice. A slide can easily start, crashing down and burying everything in its path. The breeze of air in front of the avalanche is usually very strong, strong enough to wipe out tall buildings and break glass.