Definition
To leave an aircraft in flight by parachute, typically in an emergency when the aircraft can no longer be safely flown or landed.
Plain English
Jumping out of an aircraft with a parachute when staying in it is no longer safe.
Context Anchor
Used in emergency procedures, parachute briefings, and discussions of aircraft that carry parachutes for the pilot or crew.
Derivation
From the older sense of 'bail' meaning to scoop water out of a boat. The idea carried over to aviation: when the aircraft is sinking (in trouble), the crew gets out the same way water would be removed from a foundering boat.
Why Pilots Care
Correct bailout technique and timing can mean the difference between survival and loss of life in uncontrollable situations such as structural failure or fire.
Intuition Check
Bail out does not mean canceling a flight or giving up on a plan. In aviation, it means physically leaving an aircraft in flight during an emergency.
Example Sentence 1
When the engine fire could not be controlled, the crew was forced to bail out over open country.
Example Sentence 2
The preflight briefing covered how to bail out over water and deploy the life raft afterward.