Areal density

Definition
Areal density of a given piece of storage media [is] literally, how closely bits can be packed together on a discrete surface.

Overview
Advances in areal density are what explain the astonishing rise in the capacity of hard drives, outstripping even Moore's law, which projects that the speed of microprocessors doubles every two years.

Currently, areal density on hard drives is upwards of 100 billion bits per square inch. Some scientists argue that we are approaching the superparamagnetic limit, which is the point on the nanoscale at which the physical properties of magnetic material break down &mdash; in other words, bits can only be made so small while retaining their physical properties.

Source

 * Digital Forensics and Born-Digital Content in Cultural Heritage Collections, at 5.