1 a : the hardened exterior or surface part of bread
b : a piece of this or of bread grown dry or hard
2 : the pastry cover of a pie
3 : a hard or brittle external coat or covering: as
a : a hard surface layer (as of soil or snow)
b : the outer part of a planet, moon, or asteroid composed essentially of crystalline rocks
c : a deposit built up on the interior surface of a wine bottle during long aging
d : an encrusting deposit (as of the eye) of dried secretions or exudate; also : scab
4 : gall, nerve