A cornice is an overhanging ledge or shelf of snow that usually forms over the lee side of ridges. A cornice forms as loose snow is transported over a ridge, where it collects and grows. Cornices have an unstable structure and they grow more unstable with loading or warming. When a cornice breaks, the resulting fall is usually powerful enough to trigger an avalanche.
Above: Cornices along a ridge. By Martina Halik
Below: It's important to stay far back from the edge as cornices can break surprisingly far in from the edge. By Avalanche Canada