Something described as byzantine (or Byzantine) is very complicated, secret, and hard to understand. In its most literal (and always capitalized) sense, Byzantine describes that which is "of, relating to, or characteristic of the ancient city of Byzantium."

// The company's byzantine accounting practices have gotten it into trouble with the government.

// Works of Byzantine art filled the gallery's walls.

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"Consider that California has among the strongest state privacy laws, yet tech giants such as Meta and Google, which make their homes here, are still accused of some of the most egregious privacy violations. A powerful and universal federal law is required to rein them in. It would also be more effective than the status quo of a byzantine patchwork of state laws." — Mark Weinstein, The Los Angeles Times, 29 Apr. 2024

Today, the city that lies on the Bosporus Strait in Turkey is named Istanbul, but it was once known as Constantinople (a name given to it when it became the capital of the Byzantine Empire, aka the eastern half of the Roman Empire), and in ancient times, it was called Byzantium. Its history is legendary—filled with mystics, wars, and political infighting—and over time the word Byzantine (from the Late Latin word Byzantinus, the name for a native of Byzantium) became synonymous in English with anything characteristic of the city or empire, from architecture to intrigue. The figurative sense used to describe that which is intricately involved and not easily understood first appeared in the early 20th century.