1 the hard brown outer surface of bread: If the oven is too hot the crust will burn.2the baked outer part of foods such as pies and pizzas: a pizza with a thin crust3a thin hard dry layer on the surface of something: There was a thin crust of ice on the pond.4science, earth science, physics the hard outer layer of a planet, moon, etc., made up mostly of rocks: the Earth’s crust