A physical medium for carrying signals. Fibre optics requires specially prepared optical fibres to carry light signals. Electric cable is usually insulated copper wire encountered in various forms depending on the intended application; common forms include twisted pair (unshielded and shielded), coaxial cable, and ribbon cable.

An electric circuit must always contain an outward and a return path. For low-frequency and low-power signals the outward path can consist of a single conducting wire, with the return path carried by a common ground (earth) return, which can be shared by many different circuits. At higher frequencies and powers this system is no longer effective, and it is necessary to provide both an outward and a return conductor. At still higher frequencies, the two conductors need to be kept close to one another, as in twisted pair, so that the outward current in one conductor is balanced by the corresponding inward current in the other; this reduces the amount of energy lost by radiation. Screened cable is a multipath electric cable with a surrounding screen usually formed from an interwoven fine wire mesh, and used for example in shielded twisted pair; the screen provides some isolation from external sources of electrical interference. Multicore cable is a multipath cable frequently containing a mixture of screened and single conductors; sometimes one or more coaxial cables are included to provide paths for high-frequency or other special signals.