The hard outer portion or surface area of bread. A piece of bread consisting mostly of the hard outer portion. A piece of bread that has become hard and dry. A pastry shell, as of a pie or tart. A hard crisp covering or surface: snow with a firm crust. A hard deposit formed on the interior of a wine bottle as the wine matures. Geology The exterior portion of the earth that lies above the Mohorovi\u010di\u0107 discontinuity. Geology The outermost solid layer of a planet or moon. The hard outer covering or integument of certain plants and animals, such as lichens and crustaceans. Pathology An outer layer or coating formed by the drying of a bodily exudate such as pus or blood; a scab. Informal Insolence; audacity; gall. To cover with a crust. To form into a crust. To become covered with a crust. To harden into a crust.