Simplex signaling is signaling in which two conductors are used for a single channel

Channel (communications)

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...

, and a center-tapped coil, or its equivalent, is used to split the signaling current equally between the two conductors. The return path for the current is through ground.
It is distinct from a phantom circuit

Phantom circuit

In telecommunication and electrical engineering, a phantom circuit is an electrical circuit derived from suitably arranged wires with one or more conductive paths being a circuit in itself and at the same time acting as one conductor of another circuit....

 in which the return current path for power or signaling is provided through different signal conductors.
SX signaling may be one-way, for intra-central-office use, or the simplex legs may be connected to form full duplex signaling circuits that function like composite (CX) signaling circuits with E&M lead control.

Simplex is also used to describe a powering method where one or more signal conductors carries direct current to power a remote device, which sends its output signal back on the same conductor. Phantom powering as used in audio

Sound recording and reproduction

Sound recording and reproduction is an electrical or mechanical inscription and re-creation of sound waves, such as spoken voice, singing, instrumental music, or sound effects. The two main classes of sound recording technology are analog recording and digital recording...

is a form of simplex powering, as the return current flows through the ground or shield conductor.