A hard disk is part of a unit, often called a "disk drive," "hard drive," or "hard disk drive," that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces.

a rigid magnetic disk mounted permanently in a drive unit

a flat, circular oxide-coated disc which rotates continuously

(Also "hard drive", or "hard disk" for our American friends): mass storage periphery; modern computers usually hold at least one hard disc that contains the operating system. It is also usually where user data are stored.

computer storage medium, rigid disc usually made of aluminium, coated with magnetic material and hermetically sealed; fitted internally in a computer.