Areal density is a measurement of the amount of data that can be stored on a given unit of physical space on storage media. It is most frequently measured in gigabits per square inch and used to describe hard disk drive capacity.
Some industry professionals argue that areal density steadily increases over time. This is detailed by what is known as Kryder's Law, which states that disk drive density doubles every thirteen months. While Kryder's Law is up for debate, history shows that as storage technology matures, a greater amount of data can be stored on a smaller amount of physical space.
Areal density is an important measurement for those evaluating storage options because generally, the more physical space a storage medium takes up, the more expensive it is to run.