Supersaw
Seven slightly detuned sawtooth waves crammed into one oscillator — the sound that defined trance.
Roland introduced it on the virtual-analog JP-8000 in 1996: a single oscillator that was really seven sawtooths, each pushed a few cents off pitch. Two knobs ran the whole thing, detune and mix, and the result sounded like a string section of synths that couldn't quite agree. That was the point.
Within a couple of years half the trance records on the market sounded the same, and the feature has since turned up in nearly every soft synth as unison, hypersaw or something similar. In a 2010 thesis Adam Szabó reverse-engineered the original and found that the detune curve was non-linear and that Roland high-pass filtered away the fundamental that would otherwise have muddied the sound. Which is why the clones rarely sound quite right.