Distress
Distress is defined as a condition of being threatened by serious and/or imminent danger and of requiring immediate assistance.
Distress is notified by the MAYDAY radio call, repeated three times.
Urgency
Urgency is a condition concerning the safety of an aircraft or other vehicle, or of some person on board or within sight, but which does not require immediate assistance
Urgency is notified by the spoken PAN PAN radio call, repeated three times.
Emergency Frequency
The international Aeronautical VHF Distress Frequency is 121.5 MHz. It is used as a universal emergency frequency for aircraft in distress, and is monitored by air traffic control centers and other aircraft in the area. When a pilot broadcasts a distress call on this frequency, all nearby aircraft and air traffic control centers will be alerted and can respond to the emergency.