The maximum number of bits per square inch that is commercially feasible on a magnetic storage device. As the magnetic bits get smaller, at some point they no longer hold their charge. Thermal fluctuations reduce the signal strength and render the bits unstable.
However, this maximum density limit keeps changing as researchers find new techniques for recording and sensing the bit. Years ago the limit was thought to be 20 gigabits per square inch. Today, we are already beyond a terabit per square inch. See areal density
, perpendicular recording