Crossfeed
A filter that leaks a quieter, delayed copy of each stereo channel into the opposite ear, so headphones sound more like speakers.
Stereo mixes are made for speakers, where both ears hear both speakers. On headphones the left ear hears only the left channel, and an instrument panned hard to one side ends up unnaturally inside your skull — especially on 1960s records with the drums in one channel and the vocals in the other. Benjamin Bauer described the problem, and a simple circuit to fix it, back in 1961.
Crossfeed low-pass filters each channel, since the head shadows the treble, delays it by a few tenths of a millisecond and mixes it faintly into the other side. It is a crude approximation of what a full HRTF does, but it is cheap and leaves the tonal balance largely alone. The bs2b library has put it into a good many software players.