In geometry, coaxial means that two or more three-dimensional linear forms share a common axis. Thus, it is concentric in three-dimensional, linear forms. A coaxial cable, as a common example, is a three-dimensional linear structure. It has a wire conductor in the centre (D), a circumferential outer conductor (B), and an insulating medium called the dielectric (C) separating these two conductors. The outer conductor is usually sheathed in a protective PVC outer jacket (A). All these have a common axis.