A parallel universe in which humanity built the brutal Terran Empire instead of the Federation, and where familiar officers have ruthless counterparts who rise through betrayal and assassination.
The mirror universe is introduced in the 1967 episode Mirror, Mirror, when a transporter accident swaps Kirk, McCoy, Scotty and Uhura with their counterparts aboard the ISS Enterprise. Kirk urges the bearded mirror Spock to reform the Empire.
Deep Space Nine reveals the result in Crossover and four further episodes: the weakened Empire fell to a Klingon-Cardassian Alliance and Terrans became slaves, later rising in rebellion.
Enterprise shows the Empire's early days in In a Mirror, Darkly, and the first season of Discovery spends several episodes there, bringing Emperor Philippa Georgiou and the disguised Gabriel Lorca into the story. Prodigy and the film Section 31 also draw on it.