How closely bit cells are packed together on a section of track. This is effectively determined by the bit cell width. Higher density (using bitcells of shorter width) means more data can be packed into the same length of track.
Commercial duplicator machines were able to write bit cells of different lengths. On protections where varying density was used across a track (e.g. Copylock, Speedlock, and on games like Mutant Colony and Cavitas. See our WIP entry “The wait is over!” to see visual examples), we suspect that it was written in multiple passes (for each bitcell width setting) with extremely accurate timing.
The Commodore Amiga can read disks of varying density due in part to its Jitter hardware. However there are limits of roughly +/- 15% of normal density, and anything beyond this range cannot be read on an unmodified Amiga system.