Disk capacity
Alternatively referred to as disk space, disk capacity is the total amount of bytes that a disk drive or disc is capable of holding. Typically, the storage device is the computer's hard drive, but it can also be a USB thumb drive, a CD or DVD, a memory stick/card, or a Floppy disk. Disk space is usually measured in kilobytes (KB), megabytes (MB), gigabytes (GB), or terabytes (TB). For example, a hard disk drive that is 500GB is capable of holding 500 Gigabytes of information.
As shown above, when viewing the properties of a storage device, it will usually display Used disk space and Free (Unused) disk space. Used space is what is already taken up by existing data/files on the computer and Free space is what is available to use.
Also see: Hard disk drive definition