Definition
The phase of flight in which an aircraft leaves an airport, beginning at takeoff and continuing until the aircraft has climbed out of the immediate airport environment and either joined the en route structure or transitioned to the next phase of flight. In ATC usage, 'departure' also refers to an aircraft that has just taken off and is being controlled by a Departure Control facility, and to a published procedure (a Standard Instrument Departure or Obstacle Departure Procedure) that defines the route to be flown after takeoff.
Plain English
Leaving an airport by air. The word covers the takeoff and climb-out, the aircraft itself while it's doing this, and the published route that tells the pilot how to fly out of the airport area.
Context Anchor
Seen in air traffic control instructions, flight plans, airport traffic reports, and airport information when talking about aircraft leaving an airport.
Derivation
From the Latin 'departire,' meaning 'to divide' or 'to go away from.' In aviation it keeps the everyday sense of going away from a place — specifically, going away from the airport.
Why Pilots Care
Following a proper departure keeps the aircraft clear of obstacles and terrain while establishing safe navigation to the planned route.
Intuition Check
Departure does not just mean leaving the parking area or terminal. In this FAA use, it means the aircraft’s takeoff from the airport or landing area.
Example Sentence 1
After takeoff, the tower instructed the pilot to contact Departure on 124.5 for further climb instructions.
Example Sentence 2
After takeoff the controller issued a heading for the departure that kept the flight away from arriving traffic.