Plain English Meaning

When an aircraft and air traffic control can no longer talk to each other by radio, the pilot follows a set of pre-defined rules about where to fly, how high, and when to land — so that controllers can predict the aircraft's path even without being able to speak to the pilot.

Full Definition

The loss of two-way radio communication between an aircraft and air traffic control (ATC). Under instrument flight rules (IFR), the pilot must follow specific lost communication procedures defined in 14 CFR 91.185, which prescribe the route to fly, the altitude to maintain, and when to begin the approach at the destination, depending on whether the aircraft is operating in visual or instrument meteorological conditions.