What is crost?

As an AI language model, I cannot be certain whether you meant "crost" or "crust." However, I can give you information about both:

Crost: Unfortunately, I could not find any information about "crost" as a word or term. If you could provide more context or information about what you are referring to, I may be able to assist you better.

Crust: Crust is an outer layer that forms on the surface of something. It can refer to a layer of pastry on a pie, a crunchy layer on bread, or the outermost layer of the earth. In geology, crust refers to the solid outer layer of the earth. The Earth's crust is divided into tectonic plates that move and cause earthquakes and volcanic eruptions. The thickness of the Earth's crust varies, with oceanic crust being thinner and denser than continental crust. Crust is also an important component of the lunar surface and other rocky planets and moons in the solar system.