Plain English Meaning
A radio system that lets aircraft and ground stations send each other short digital messages — including position reports — over a shared VHF channel, with each station automatically taking turns so messages don't collide.
Full Definition
A digital data link that uses VHF radio frequencies to exchange short messages between aircraft, ground stations, and other aircraft. Mode 4 uses a self-organizing time-division access scheme, meaning each station automatically picks an unused time slot to transmit, allowing many users to share the same frequency without a central controller. It supports air traffic services, (including reporting similar to ), and aircraft-to-aircraft .