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3.5 inch(1) Storage drives and other devices with a 3.5" form factor. The outside case measurement of the 3.5" drive, which is commonly used in desktop computers, is approximately 4" wide (100mm). See form factor.
(2) The 3.5" microfloppy disk or the 3.5" microfloppy drive that came with all computers until the late 1990s. See floppy disk.