Starfleet Records. Mirror universe. Meanwhile, in the prime universe, Mr. Spock had realized that Captain Kirk came from an alternate reality that was not theirs. Source: TOS: "Mirror, Mirror." Star Trek. Live the experience. | Starfleet Command

Starfleet Records. Mirror universe. The mirror universe is so called because most of the places, ships, and people that exist in the main universe also exist there, although they generally present the antithesis of many of its personal characteristics, in particular exchanging its morally good characteristics (for example: altruism, goodness), for morally evil characteristics (for example: selfishness, cruelty) or vice versa, therefore, Starfleet has called these universes "mirror". Sources: DIS

Starfleet Records. Mirror universe In 2267, four Starfleet officers (Captain James T. Kirk, Lieutenant Uhura, Lieutenant Commander Montgomery Scott, and Doctor Leonard McCoy) from the USS Enterprise were exchanged with those same officers from the ISS Enterprise in the mirror universe because of transporter interference from an ion storm. They discovered a brutal regime, almost dictatorial in its command structure. Advancement through assassination was commonplace. Source: TOS: "Mirror, Mirror".