The number of bits per square inch of storage surface. It typically refers to disk drives, where the number of bits per inch (bpi) times the number of tracks per inch (tpi) yields the areal density.

**From Two Thousand Bits to One Trillion**

The areal density of disk storage devices has increased dramatically since IBM introduced the RAMAC, the first hard disk computer in 1956. RAMAC had an areal density of two thousand bits per square inch, while current-day disks have reached one terabit (1 trillion bits per square inch). See perpendicular recording, superparamagnetic limit, holographic storage and AFM.

The areal density of disk storage devices has increased dramatically since IBM introduced the RAMAC, the first hard disk computer in 1956. RAMAC had an areal density of two thousand bits per square inch, while current-day disks have reached one terabit (1 trillion bits per square inch). See perpendicular recording, superparamagnetic limit, holographic storage and AFM.

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