1. An external coat or covering of a thing, which is hard or harder than the internal substance; as the crust of bread; the crust of snow; the crust of dross; the crust of a pie.

2. A piece of crust; a waste piece of bread.

3. A shell, as the hard covering of a crab and some other animals.

4. A scab.

5. The superficial substances of the earth are, in geology, called its crust.

, v.i. To gather or contract into a hard covering; to concrete or freeze, as superficial matter.