Definition
LOST COMMUNICATIONS
Loss of the ability to communicate by radio. Aircraft are sometimes referred to as NORDO (No Radio). Standard pilot procedures are specified in 14 CFR part 91. Radar controllers issue procedures for pilots to follow in the event of lost communications during a radar approach when weather reports indicate that an aircraft will likely encounter IFR weather conditions during the approach.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).