areal density

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 2,000 to 100,000,000,000 Bits
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 100 billion bits (100 gigabits per square inch). Perpendicular recording is expected to increase storage capacity even more dramatically. See perpendicular recording, superparamagnetic limit, holographic storage and AFM.

Rings on Dumbbells for Denser Memory
The areal density of memory chips is much greater than that of magnetic disks. However, in 2006, the California Institute of Technology built a prototype memory chip that used molecular switches resembling dumbbells with a ring that slides along the center bar. Voltage pushes the ring from one end to the other to represent a 0 or 1. Only in the early stage of development, the first prototype chip was no larger than a white blood cell and held 160,000 bits. If it were to become commercially viable, the density would exceed 600 billion bits per square inch. In addition, scientists claimed that improvements in this technology are expected that could make the bits 10 times denser. Such an achievement would yield six trillion bits per square inch!