n. A hard external portion, of comparative thinness, forming a sort of coating over the softer interior part; any hard outer coat or coating: as, the crust of frozen snow; the crust of a loaf of bread; a thin crust of politeness.n. Specifically In geology: The exterior portion of the earth; that part of the earth which is accessible to examination.n. The solid portion of the earth, as opposed to its fused interior, many geologists and physicists believing that the interior of the earth must be in a more or less fluid condition.n. Matter collected or concreted into a solid body; an incrustation; specifically, a deposit from wine, as it ripens, collected on the interior of bottles, etc., and consisting of tartar and coloring matter.n. A piece of an outer coating or incrustation; specifically, an external or a dried and hard piece of bread.n. In zoology, a shell; a test; the chitinous or other hard covering of various animals, as crustaceans and insects.n. In anatomy and physiology, a coat or covering harder or denser than that which is covered; a pellicle; a crusta: as, the buffy coat or crust of inflammatory blood; the crust of a tooth.n. The part of the hoof of a horse to which the shoe is fastened.To cover with a crust or hard exterior portion or coating; overspread with anything resembling a crust; incrust.To coat or line with concretions. See crust, n., 3.To thicken or contract into a hard covering; concrete or freeze, as superficial matter.To crust-hunt.