Definition
EMERGENCY LOCATOR TRANSMITTER (ELT)
A radio transmitter attached to the aircraft structure which operates from its own power source on 121.5 MHz and 243.0 MHz. It aids in locating downed aircraft by radiating a downward sweeping audio tone, 2‐4 times per second. It is designed to function without human action after an accident.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).