A slab avalanche of soft or low density snow.
Soft Slabs:

Although soft slabs are defined to be slabs less than 30 percent water weight (300 kg/m3), there is a rather fuzzy boundary between hard slabs and soft slabs. We generally think of soft slabs as composed of new snow while hard slabs are usually composed of old, denser layers of snow or very wind-hardened new snow. Also, soft slab avalanche debris tends to be soft and powdery while hard slab debris often contains chunks hard snow.

The most important distinction for people in avalanche terrain is that soft slabs tend to break at or near your feet while hard slabs tend to break above you, making them much more difficult to escape being caught.

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