Plain English Meaning
A switch in the cockpit that picks which radio your microphone will talk through when you press the push-to-talk button.
Full Definition
A cockpit control, typically a rotary switch on the audio panel, that determines which radio transmitter is connected to the pilot's microphone. When the pilot keys the mic, the audio is sent out through whichever radio the transmitter selector is currently pointing to.
Context Anchor
Seen on aircraft audio panels and communication radio controls when choosing which radio to use for a call.
Derivation
“Transmitter” comes from a word meaning “to send across.” “Selector” means something that chooses. Together, the term points to a control that chooses which radio will send your voice out.