A slab avalanche of soft or low density snow.
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.