A type of electronic dance music characterised by a moderately fast tempo (typically 140-155 bpm) and "steady 4" rhythm (as opposed to the more syncopated rhythms commonly found in house, breakbeats, etc.)
Trance has a strong influence from early Detroit techno, but tends to be more dramatic or "emotional" than the minimalism of earlier techno. Common elements in trance include heavy, compressed kick drums, dramatic sweeping pads, and cascading synth arpeggios.
As with any kind of music, there are a billion sub-classifications possible -- hard trance, psytrance, acid trance, goa trance, etc. Trance has been one of the most popular forms of club music over the past few years, and has arguably produced more mainstream appeal (Oakenfold, Van Dyk, etc.) than most other electronic sub-genres.
"She spun a sick trance set in the big room last night; lots of distorted 909s and squelchy basslines, none of this shite that sounds like a car commercial."