What overall episode number was SVU's "Authority"?

SVU's "Authority" was the show's 200th overall episode.

The episode is the seventeenth installment of season 9 and first aired on April 29, 2008. Its milestone status helped attract Robin Williams, who guest-starred as Merritt Rook, an audio engineer who manipulates people by pretending to be a police officer. The story follows Stabler and Benson as they investigate the consequences of his calls.

The episode's production team treated the landmark as an opportunity to build a high-profile guest role rather than simply making the number part of the plot. Williams's casting was reported before broadcast, and Richard Belzer's connection to him helped open the door. "Authority" also drew on real incidents and psychological studies, including the Mount Washington strip-search scam and the Milgram experiment.

The number is sometimes confused with the season or episode number: it is season 9, episode 17, but episode 200 across the entire series. That distinction is why 200th is the canonical answer.