A snowpack layer with less strength than adjacent layers. Often the layer in the snowpack where an avalanche fractures.
All snow exists as layers. Some layers are relatively more cohesive (stronger layers) and others are relatively less cohesive (weaker layers). Like cliffs in the Grand Canyon, if you brush a snowpit wall with your mitten, the weaker layers erode away while the stronger layers stick out. When the snowpack is stressed by rapid changes (e.g. wind-drifted snow, new snow, or rain) this stress can cause the weak layer to fracture. Understandably, most avalanche geeks are obsessed by weak layers. Weak layers involved in most avalanche accidents usually are a “persistent” grain type such as faceted snow, surface hoar or depth hoar, but it can also be a layer formed within new snow such as low density new snow or graupel. After an avalanche occurs, you often hear avalanche professionals ask, “what was the weak layer” or “what did it run on?” The life of an avalanche aficionado revolves around knowing what is the weakest layer in the snowpack and how much stress it takes to make it fail.