In which episode did the Borg first appear?

The Borg first appeared in the Star Trek episode “Q Who.”

The episode is the 16th installment of the second season of Star Trek: The Next Generation, originally broadcast in 1989. Q transports the Enterprise-D far across the galaxy to demonstrate that Captain Jean-Luc Picard and his crew are not prepared for every danger beyond Federation space. There, they encounter the cybernetic Borg for the first time.

The Enterprise tries to communicate with the Borg and later attacks their cube, but the crew quickly discovers that conventional weapons are ineffective. The Borg begin cutting into the ship and assimilating its systems. Picard finally asks Q for help, and Q returns the Enterprise to its original location, warning that the encounter was only a preview of threats ahead.

“Q Who” is often confused with “The Best of Both Worlds,” the later two-part story in which Picard is assimilated as Locutus. “First Contact” also features the Borg prominently, but it is a 1996 film rather than their first appearance. The Borg’s planned introduction in “The Neutral Zone” was postponed during production.