from 330 C.E. to 1930 the name of what is now Istanbul and formerly was Byzantium, the city on the European side of the Bosphorus that served as the former capital of the Byzantine and Ottoman empires, from Greek Konstantinou polis "Constantine's city," named for Roman emperor Flavius Valerius Aurelius Constantinus (see Constantine), who transferred the Roman capital there.
"harem," also the name of a former palace of the sultan in Istanbul, 1580s, from Italian seraglio, alteration of Turkish saray "palace, court," from Persian sara'i "palace, inn," from Iranian base *thraya- "to protect" (source also of Avestan thrayeinti "they protect"), from PIE *tra-, variant form of root *tere- (2) "cross over, pass through, overcome."
The Italian word probably reflects folk etymology influence of serraglio "enclosure, cage," from Medieval Latin serraculum "bung, stopper" (see serried).
late 14c., "the direction east; the part of the horizon where the sun first appears," also (now with capital O-) "the eastern regions of the world, eastern countries" (originally vaguely meaning the region east and south of Europe, what is now called the Middle East but also sometimes Egypt and India), from Old French orient "east" (11c.), from Latin orientem (nominative oriens) "the rising sun, the east, part of the sky where the sun rises," originally "rising" (adj.), present participle of oriri "to rise" (see origin).
Meaning "a pearl of the first water" is by 1831, short for pearl of the Orient (late 14c.) originally meaning one from the Indian seas. Hence also the meaning "a delicate iridescence, the peculiar luster of a fine pearl" (1755). The Orient Express was a train that ran from Paris to Istanbul via Vienna 1883-1961, from the start it was associated with espionage and intrigue.