Standard Instrument Departure
A Standard Instrument Departure (SID) is an air traffic control coded departure procedure that has been established at certain airports to simplify clearance delivery procedures. SIDs are supposed to be easy to understand and if possible limited to one page. Although a SID will keep aircraft away from terrain, it is optimized for ATC route of flight and will not always provide the lowest climb gradient, but strike a balance between obstacle avoidance and airspace considerations. In order to legally fly a SID the pilot must possess at least the textual description of the SID procedure. 
SIDs start at the DER (Departure End of Runway) and leads along waypoints (VOR, NDB, DME) on defined flight levels. The end of a SID is reached when the aircraft is leaving the surveyed airspace of the departed airport's ATC. The SID system exists to ensure secure passing of obstacles along the flight path. Defining a SID relies mainly on noise-avoiding and security aspects and can reflect noise-avoiding at different times of day above cities or secure passing along important installations (military, private, public, governmental, etc.).