From AMS Glossary
A disturbance in a snowfield caused by the simultaneous settling of a large area of thick snow crust or surface layer.
It occurs when wind action has maintained the top foot or more as closely packed, fine-grained snow rather impervious to air movement. Meanwhile, at lower depths, firnification has caused the larger crystals (depth hoar) to grow at the expense of smaller ones, creating air pockets and a weak structure. The collapse of this structure (a perceptible drop, but rarely as much as 1 in.) may be accompanied by a loud report; over a large, level field, adjacent patches may settle as a series of tremors. Occasionally, a snow geyser may be blown upward through the crack of a settling patch.