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Areal density is the measurement of storage units per square inch, or, more generally, the measurement of storage capacity relative to physical dimensions.
Areal density is sometimes used interchangeably with the terms area density and surface density.
Areal density is an important idea in IT. For an easy example of areal density, think of a small piece of storage media. Say the disk is one square inch in terms of its physical size. If that disk has 1 GB of storage capacity, the areal density measurement is 1 GB per square inch.
Areal density is a useful term in looking at the relative storage capacity of physical storage media like magnetic tapes or disks and optical disks. Reports from the IT industry over time have shown how areal density improves at a dramatic rate. The ability to pack more digital storage capacity into smaller disks and devices is a large part of what has driven improvement in the hardware industry and other parts of the tech business.