The concept of extraterritoriality is a fiction in international law which has been used to account for immunities which offer a way for certain persons or certain things (diplomatic agents and premises) to escape from the authority of the State of residence as if they were within national territory.
Source: Lexique des termes juridiques (ISBN 2 247 03487 9), Dalloz, 12th edition 1999, pages 241 to 242.