Plain English Meaning

It means the radio isn't working well enough to talk to or hear ATC, so the pilot has to follow a set of pre-agreed rules for what to do next instead of getting instructions over the air.

Full Definition

An in-flight condition in which the aircraft's two-way radio communication with air traffic control is lost, requiring the pilot to follow specific lost communication procedures published in the Federal Aviation Regulations and on the relevant approach or en route charts.

Context Anchor

Seen in instrument flying material and discussions, especially when describing what a should do after losing with air traffic control.