Areal density
Bits per unit area on a platter — the hard disk's headline progress metric, the product of linear density and track density.
Areal density is the product of linear density (bits per inch along the track) and track density (tracks per inch across it). It was the number that drove the whole HDD industry: IBM's RAMAC stored roughly 2 kbit per square inch in 1956, while modern drives pass a terabit per square inch.
For a stretch it grew faster than Moore's law — sometimes called Kryder's law. The ceiling is set by the superparamagnetic limit, and every fresh barrier broken (PRML, PMR, HAMR) has been about pushing that figure further.