Which original-series episode introduced Khan Noonien Singh?

The original-series episode that introduced Khan Noonien Singh was “Space Seed.”

“Space Seed” is the 22nd episode of Star Trek: The Original Series' first season. It first aired on February 16, 1967, and guest-starred Ricardo Montalbán as Khan. The Enterprise discovers the SS Botany Bay, a derelict sleeper ship carrying genetically engineered humans from Earth's past in suspended animation.

Khan was revealed to be a former conqueror from the Eugenics Wars of the 1990s. After awakening, he attempts to seize control of the Enterprise, believing that his superior abilities entitle him to command. Captain Kirk ultimately defeats him and maroons Khan and his followers on Ceti Alpha V.

The episode became much more influential after Khan returned in Star Trek II: The Wrath of Khan (1982). That film transformed a memorable guest villain into one of the franchise's defining antagonists. A famous continuity detail is that Khan recognizes Chekov, although Chekov was not yet part of the televised crew when “Space Seed” was produced.