A snowpack layer with less strength than adjacent layers. Often the layer in the snowpack where an avalanche fractures.
Weak Layer:

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.

Additional Terms:
Anchors Hard Slab Avalanche Slide
Aspect High Danger Sluff
Avalanche High Marking Snowpit
Avalanche Path Isothermal Soft Slab Avalanche
Avalanche Transceiver Layer, Snow Stability
Bed Surface Leeward Stability Test
Collapse Loading Starting Zone
Concave Slope Loose Snow Avalanche Stepping Down
Considerable Danger Low Avalanche Hazard Sun Crust
Convex Slope Melt-Freeze Snow Surface Hoar
Cornice Metamorphism, Snow Sympathetic Trigger
Corn Snow Moderate Danger Temperature Gradient
Couloir Persistent Weak Layers Terrain Trap
Cross Loading Point-Release Track
Crown Face Probe Trigger
Danger Ratings Propagation Trigger Point
Deep Slab Avalanche Rain Crust Upside-Down Storm
Density, Snow Remote Trigger Weak Layer
Depth Hoar Rime Weak Interface
Dry Snow Avalanche Runout Zone Wet Snow Avalanche
Extreme Danger Sastrugi Windward
Faceted Snow Settlement Wind Loading
Fracture Ski or Slope Cut Wind Slab
Glide Skinning, Skin Track Whumpf
Graupel Slab