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5.25 inch(1) Storage drives and other devices with a 5.25" wide form factor. The outside case measurement of the 5.25" drive, which is commonly used for optical discs, is approximately 5.75" wide (146mm). The 5.25" form factor was also widely used for hard disks before the arrival of the 3.5" drive.
(2) Refers to the earlier 5.25" floppy disk that was widely used in personal computers until the early 1990s. It gave way to the 3.5" floppy, which itself became antiquated by the turn of the century. See floppy disk.