Definition
A method of radio communication in which transmission and reception occur on the same frequency, and only one party can transmit at a time. While one station is talking, the other must listen and wait until the channel is clear before responding.
Plain English
One frequency, one talker at a time. You speak, release the mic, then the other person speaks. Both sides cannot talk at once.
Context Anchor
Seen in radio communication discussions, especially when describing how pilots and air traffic control use the same frequency for a call.
Derivation
From the Latin 'simplex,' meaning 'simple' or 'single.' The term reflects the single-channel, one-direction-at-a-time nature of the communication.
Why Pilots Care
Almost all aviation voice radio is simplex. Stepping on another transmission (talking while someone else is) blocks both messages and can cause missed instructions or traffic calls. Knowing the channel is shared keeps pilots disciplined about listening before transmitting.
Intuition Check
Simplex does not just mean “simple” or “easy” here. It means the same radio frequency is used for both talking and listening.
Example Sentence 1
Because the CTAF is a simplex frequency, the pilot waited for the inbound traffic to finish their call before announcing his own position.
Example Sentence 2
In simplex operation the controller waited for the pilot to finish before transmitting a reply.