hidden-node problem
Two devices that can both reach the access point but can't hear each other, so their transmissions collide.
In the hidden-node problem two stations out of range of each other both sense a quiet channel and transmit to the same access point, colliding there; RTS/CTS and placement reduce it.
Examples
Two sensors at opposite ends of a greenhouse both reach the router but can't hear each other, so they transmit at once and collide at the router.
RTS/CTS fixes hidden nodes: the sender asks the access point for the air first, and the AP's reply silences everyone who can hear it.
See also