A casing is an outer covering that protects or contains something inside. The plastic shell around your phone is a casing that keeps the delicate electronics safe from drops and bumps. Sausages have casings made from thin material that holds the meat together during cooking. Wells drilled deep into the earth use metal casings, strong pipes that prevent the walls from collapsing inward.

Bullets have brass casings that hold the gunpowder. Architects design door and window casings, decorative frames that go around doorways and windows to cover the rough edges where the wall meets the opening.