1. a. The hard outer portion or surface area of bread. b. A piece of bread consisting mostly of the hard outer portion. c. A piece of bread that has become hard and dry.

2. A pastry shell, as of a pie or tart.

3. A hard, crisp covering or surface: snow with a firm crust.

4. A hard deposit formed on the interior of a wine bottle as the wine matures.

5. Geology. a. The exterior portion of the earth that lies above the Mohorovičić discontinuity. b. The outermost solid layer of a planet or moon.

6. The hard outer covering or integument of certain plants and animals, such as lichens and crustaceans.

7. Pathology. An outer layer or coating formed by the drying of a bodily exudate such as pus or blood; a scab.

8. Informal. Insolence; audacity; gall.

1. To cover with a crust.

2. To form into a crust.

1. To become covered with a crust.

2. To harden into a crust.