A relatively cohesive snowpack layer.
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What makes a slab?

When stronger snow overlies weaker snow, we call it a slab. Or as Karl Birkeland puts it, "A slab is when you have something sitting on top of nothing." A slab can occur anywhere in the snowpack but avalanche professionals usually think of a slab as the layer that slides off the slope to create the avalanche.

Remember that a slab doesn't have to be so hard that you can hardly kick you boot into it. It just has to be relatively stronger than the snow underneath. Light, dry powder snow can behave as a slab as long as it has an even weaker layer underneath.

Most commonly, slabs tend to be harder, more cohesive snow such as wind slabs, denser new snow or settled old snow.
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