Acronym meaning: Emergency Locator Transmitter.

ELT's are devices carried by military aircraft, (some civilian as well,) that when activated by a sudden deceleration broadcast a continuous signal at 243.0 MHz and 121.5 MHz. This is designed to allow {SAR|search parties] and recovery teams to locate the downed aircraft faster in the event of a mishap or crash requiring outside assistance that rendered the radios unusable.

243.0 MHz and 121.5 (notice one is half of the other,) are commonly referred to as 'Guard' and are the most commonly used international distress frequencies.

These differ from the Underwater Acoustic Beacons carried by most aircraft in that they use RF and not sound as a primary means of transmission.
UAB's or 'pingers' (as they are more commonly referred) on the other hand, are not impact activated but by a plug dissolved by salt water. These dissipate after 4-10 days and then emit a pulse every second for several days following.

Author's Note: The pinger on a UAB is similar to the one's used on Exercise Torpedoes. We had one of these inadvertently start while onboard a ship during RIMPAC '98, you could hear the thing through two inches of steel and it was very annoying. It bears mentioning that on a Perry Class Frigate the torpedo magazine is just forward of the aviation maintenance shop, DET 8I was lucky enough to listen to this torp chirping for about three weeks.

As a second note and to add to the collective peace of mind: Typically we have between ten and fifteen inadvertent (read: maintenance error) ELT activations a year at NAS North Island, and every single one is taken just as seriously as if it were an actual mishap.