Definition
The international VHF aeronautical emergency frequency, 121.5 MHz, monitored by air traffic control facilities, many military and civilian aircraft, and search and rescue services. It is reserved for distress, urgency, and emergency communications, and is the standard frequency to call on when a pilot is unsure of the correct frequency, has lost communication with ATC, or needs immediate assistance.
Plain English
121.5 MHz is the radio channel that everyone listens to for emergencies. If you are in trouble or cannot reach anyone on your normal frequency, call on Guard and someone will hear you.
Context Anchor
You may see or hear this term when setting a radio, monitoring an emergency frequency, responding to a lost-communications situation, or following procedures after an abnormal event.
Derivation
Called "Guard" because controllers, military units, and other aircraft "guard" (continuously monitor) the frequency for anyone in trouble. The word "guard" here means "to keep watch over," the same sense as a guard standing watch.
Why Pilots Care
Missing an emergency call on guard can delay rescue or create airspace conflicts; the requirement exists so that every aircraft can serve as a relay or first responder if needed.
Intuition Check
Guard does not mean a security person here. It means the watched aviation emergency radio frequency, 121.5 MHz.
Example Sentence 1
After losing radio contact with the center controller, the pilot switched to 121.5 and transmitted in the blind on Guard.
Example Sentence 2
While flying over remote terrain, the crew kept one radio on guard in case another aircraft declared an emergency.