In espionage operational techniques, a chief of station may be known as a resident. There may also be multiple chiefs; for instance, Russia probably still operates GRU military and SVR civilian. There are two types of resident spies: legals and illegals. In Soviet usage, the operation controlled by the rezident was a rezidentura.
A legal resident is a spy who operates in a foreign country under diplomatic cover (e.g. from his country's embassy). He is an official member of the consular staff, such as a commercial, cultural or military attaché. Thus he has diplomatic immunity from prosecution and cannot be arrested by the host country if suspected of espionage. The most the host country can do is send him back to his home country.
An illegal resident is a spy who operates in a foreign country but without the cover of his embassy. Thus he cannot claim immunity from prosecution when arrested. He usually operates under a false name and has documents making him out to be an actual national or from a friendly country. Examples of such illegals include Rudolf Abel who operated in the United States; and Gordon Lonsdale who was born in Russia, claimed to be Canadian and operated in Britain. Famous Soviet "illegals" include Richard Sorge, Walter Krivitsky, and Alexander Ulanovsky.