Definition
A method of two-way radio communication in which a single frequency is shared by both stations, allowing only one party to transmit at a time while the other listens. The operator must release the transmit key before the other station can reply.
Plain English
A way of talking on the radio where only one person can speak at a time. You press the button to talk, let go, then the other person can speak. Both sides take turns on the same channel.
Context Anchor
Seen in radio communication discussions, especially when describing systems where the microphone button must be released before the other person can answer.
Derivation
Half' means partial or one-sided. 'Duplex' comes from Latin 'duplex' meaning 'twofold' or 'double.' Together, 'half-duplex' describes a two-way system that only works in one direction at a time -- so it's two-way, but only halfway at any given moment.
Why Pilots Care
Pilots must release the push-to-talk switch to hear responses, preventing missed instructions or blocked transmissions.
Analogy
It works like a walkie-talkie: push to talk, release to listen. A normal phone call, by contrast, lets both people talk at once -- that is full-duplex.
Intuition Check
Half-duplex does not mean the radio only works halfway. It means two-way communication is possible, but only one direction at a time.
Example Sentence 1
Because aircraft radios use half-duplex operation, the pilot waited until the controller finished speaking before keying the mic to reply.
Example Sentence 2
Half-duplex operation on the common traffic advisory frequency requires pilots to wait until the channel is clear before transmitting.