Definition
The combined identifier used to file and fly a Standard Instrument Departure (SID), consisting of the published departure procedure's name and the specific transition route that connects the end of that departure to the en route structure. The departure name identifies the procedure itself; the transition identifies which published route the pilot will fly from the end of the common departure portion out to a specific en route fix or airway.
Plain English
When you fly a published departure, it has two parts written together: the name of the departure and the name of the route that takes you from the end of it to where your en route flying begins. You file and read back both parts as one.
Context Anchor
Seen on instrument departure charts and in air traffic control clearances for instrument flights.
Derivation
"Transition" comes from the Latin transitio, meaning a passing across or moving from one place to another. On a departure, the transition is literally the segment that moves the aircraft from the end of the departure procedure across to the en route structure.
Why Pilots Care
Ensures the aircraft follows a safe, obstacle-cleared path from the runway to the en route phase while matching the ATC clearance exactly.
Intuition Check
Do not read transition here as a general change or phase of flight. In this context, it means a specific named route option attached to a published departure procedure.
Example Sentence 1
ATC cleared the flight via the "ANDYY THREE departure, GIPER transition," so the crew flew the ANDYY THREE procedure and then continued on the published GIPER route to join the en route structure.
Example Sentence 2
ATC issued the clearance using the published departure name and transition to connect the aircraft to the airway.