A high-density diskette, or HD disk, is a 3.5" diskette capable of holding up to 1.44 megabytes of data or a 5.25" diskette capable of holding up to 1.2 megabytes of data. The additional data capacity is achieved by reading and writing to both sides of the disk.
A high-density diskette, also sometimes called a double-sided disk, is distinguished from others by its label, which indicates 1.44 or 1.2 megabytes capacity. It is also frequently identified by the letters HD printed on the diskette case. Today, if a computer store still carries floppy diskettes, they are only going to have high-density diskettes and not the earlier double density diskettes.