In the Star Trek universe, the alternate dimension where events have taken a drastic change from the events in 'our' Star Trek universe. A continuum parallel to and coexisting to our own, but on another dimensional plane. Everything in the mirror universe is duplicated, but in many cases is opposite in nature to its counterpart in our own universe. Captain Kirk and a few members from the Enterprise were thrust into this parallel existence after beaming during an ion storm, finding the mirror universe to be a brutally savage place. Spock's counterpart in the mirror universe believed his brutally oppressive government (see GOP) would inevitably spur a revolt, resulting in a terrible dark age. The mirror Spock indicated a willingness to help reform his governent to possibly avert this sequence of events.
The story for the mirror universe was re-introduced into Star Trek during the run of Deep Space Nine. Using the transporter as a catalyst for the crossover, several members of the Deep Space Nine crew were transported to the mirror universe. They found that the savage oppressive Terrans has been transformed into slaves of the Klingon-Cardassian-Bajoran union, quite the change from Kirk and Spock's time.
author note: to be revised/added to shortly. 11/06/02