A relatively cohesive snowpack layer.
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.