A single-wire transmission line (or single wire method) is a method of transmitting electrical power or signals using only a single electrical conductor. This is in contrast to the usual use of a pair of wires providing a complete circuit, or an electrical cable likewise containing (at least) two conductors for that purpose.
The single-wire transmission line is not the same as the single-wire earth return system, which is not covered in this article. The latter system relies on a return current through the ground, using the earth as a second conductor between ground terminal electrodes. In a single-wire transmission line there is no second conductor of any form.