Plain English Meaning
The tools and services that help find a pilot quickly if something goes wrong. Some are built into the aircraft (like a beacon that switches on after a crash), and some are services pilots sign up for that watch their flight and call for help if the aircraft goes down or stops reporting.
Full Definition
The combination of onboard equipment, procedures, and external services that locate and assist an aircraft following an accident, forced landing, or other in-flight emergency. Common components include the Emergency Locator Transmitter (ELT), 406 MHz satellite-based distress beacons, and subscription-based flight following or tracking services that alert search and rescue authorities when an aircraft is overdue or signals distress.