A slab avalanche of hard, dense snow. Slab density in hard slabs is typically at least 300 kg/m3.
Hard slabs are stiff, cohesive slabs, usually deposited by strong wind drifting or the slabs may be old, hardened layers of snow. Think of them like a pane of glass on top of potato chips. The good news is that hard slabs are more difficult to trigger than soft slabs, but the bad news is that they tend to propagate farther and make a much larger and more deadly avalanche. Also, the stiffer the slab, the farther above you the fracture line will usually form, and the harder it will be to escape.
Hard slabs are especially tricky because the stiffness and/or thickness of slabs can vary a lot from place to place, so just because you may not be able to trigger a slab in a thick spot, as soon as you get to the edge of the slab–for instance where it may thin near a ridgeline–you may be able to trigger the whole slope.