A crust is a hard outer layer that covers or surrounds something.
When we talk about food, the crust is often the crunchy outside part of a loaf of bread, a pizza, or a pastry. Imagine biting into a freshly baked baguette - the crispy edge is the crust. But "crust" can also refer to the dried-out top layer on the surface of soil or a body of water, like mud that's formed from sediment at the bottom of a lake.
How common is "crust"?
Word crust is considered uncommon in modern English. It appears most frequently in magazine texts and less often in other writings.
Definitions
noun
A more solid, dense or hard layer on a surface or boundary.
The external, hardened layer of certain foodstuffs, including most types of bread, fried meat, etc.
An outer layer composed of pastry
The bread-like base of a pizza.
(British, Australia, New Zealand, Ireland, Canada, Northern US) A slice of bread cut from the end of a loaf; the heel.
(geology) The outermost layer of the lithosphere of the Earth.
Example: Holonyms: lithosphere < Earth
(astronomy, by extension) The outermost layer of the lithosphere of any terrestrial planet.
Example: Holonyms: lithosphere < planet
The shell of crabs, lobsters, etc.
(British, Australia, informal) A living.
Example: to earn one's crust
(uncountable, informal) Nerve, gall.
Example: You've got a lot of crust standing there saying that.
(UK, Australia, slang, dated) The head.
(music) Ellipsis of crust punk, a subgenre of punk music.