hd - Computer Definition
(2) (High Definition) Typically refers to high-definition in the digital domain (for details, see HDTV). In the past, HD referred to several proprietary high-resolution video formats, both analog and digital, that exceeded the resolution of analog TV (NTSC, PAL). See HD formats, HD Radio, HD Audio and HDV.
(3) (Hard Disk, Hard Drive) The HD acronym is widely used in specifications; for example, 500GB HD (or 500GB HDD) refers to a 500 gigabyte hard disk. FD means floppy disk; now a distant memory. See magnetic disk.
(4) (High Density) An earlier designation for common floppy disk formats. For example, the 1.44MB 3.5" floppy was HD in contrast to the earlier double density (DD) 720KB format. For 5.25" floppies, the 1.2MB version was HD; the earlier 360KB was DD. See floppy disk.