BPI (bits per inch)
The number of bits packed along one inch of a track — one of the two axes that together make up areal density.
Storage density is measured in two directions: BPI along the track and TPI across it (track density). Multiply them and you get areal density, bits per square inch, which is the figure the marketing chases. BPI is tied to how many flux reversals the medium can hold per inch, and this is where encoding matters — FM, MFM and RLL each pack more data bits per reversal.
Historically BPI went from a few hundred on 1950s drums to millions on modern platters. Zone bit recording admits that the outer tracks are physically longer and puts more sectors there, precisely to keep BPI near the medium's ceiling across the whole disk instead of wasting the outer rim.