When using VHF radio, what call is reserved for distress situations that require immediate assistance?

The term reserved for distress situations that require immediate assistance is "MAYDAY." This is the internationally recognized distress signal used over radio communications to indicate a life-threatening emergency. When a "MAYDAY" call is made, it signifies that a vessel or person is in imminent danger and needs urgent help, thus prompting immediate response from any nearby vessels or authorities.

In contrast, "PAN PAN" indicates an urgency but not an immediate life-threatening situation. It is used for situations that require attention but do not pose an immediate threat to life. "SECURITY" is used for safety messages, often related to navigational hazards and is not meant for distress. The word "HELP" is not a formal call and can cause confusion, as it does not convey the urgency needed in maritime communications. Therefore, "MAYDAY" is the appropriate and accurate signal for distress that demands immediate assistance.