Plain English Meaning
A published, named departure route that pilots fly after takeoff to safely transition from the airport into the wider air traffic system, instead of getting a long set of individual instructions from controllers.
Full Definition
A preplanned instrument flight rules (IFR) departure procedure published in graphic and textual form, providing pilots with a charted route from the runway to the en route structure. SIDs are designed to simplify clearance delivery, reduce radio congestion, and ensure obstacle clearance and traffic separation during the climb-out from busy airports.