Plain English Meaning
An emergency beacon that pops free of the aircraft during a crash or water landing and starts sending a distress signal on its own.
Full Definition
A category of emergency locator transmitter (ELT) that is mounted to the aircraft's exterior and automatically ejects from the airframe upon impact or, in the case of water, when activated by a hydrostatic sensor. Once deployed, the unit operates independently of the aircraft and transmits a distress signal on the designated emergency frequencies for search and rescue.
Context Anchor
Seen in discussions of emergency locator transmitter types and aircraft emergency equipment requirements.
Derivation
Deployable' comes from the French déployer, meaning 'to unfold or release.' Here it signals that the unit is designed to physically separate from the aircraft, rather than stay bolted in place like the more common fixed ELT.