Half duplex

A half duplex (often written half-duplex) is a communication system or medium that physically permits transmission only in one direction, at any given time, between endpoints. To understand that the ability to have physical two-way communications but not a usable two-way flow, consider the example of two people, talking face-to-face through the medium of air. While they can both talk at the same time, the conversation will not be intelligible.

Two-way alternating
Two-way alternating (TWA) systems do not require full duplex medium. This term refers to the logical level of communications, such as the need to talk, and then to listen, to have an intelligible face-to-face conversation.

TWA can also be defined on a full duplex medium.

Simplex
Simplex communications can flow only in one direction over a telecommunications medium, such as television or radio broadcasts.