An Avalanche is a mass of snow which comes out and slides down after a break in the snowpack. Avalanches release (spontaneously or artificially) by either an increase in stress (e.g. by a skier) and/or a decrease in strength (e.g. by warming or rain). They may occur wherever there is a slope of sufficient incline and snow cover. Be aware that a slope may slide with a small additional load.
Most of the avalanches caused by winter recreationists are snow slab avalanches. Cohesive snow on top of a weak layer: The slab breaks away as a unit. Dry snow slabs are the most dangerous avalanches for winter recreationists. They are also possible when the surface layers seem to consist of cohesion less powder snow.
Each year, many people are killed by avalanches, avalanche awareness is therefore a basic skill needed for backcountry travel.