Disk Defragmenter is a utility in Microsoft Windows designed to increase access speed by rearranging files stored on a disk to occupy contiguous storage locations, a technique called defragmentation. Defragmenting a disk minimizes head travel, which reduces the time it takes to read files from and write files to the disk. Beginning with Windows XP, Disk Defragmenter also reduces system startup times. As of Windows 10, the program has been renamed to Defragment and Optimize Drives.
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https://www.computerhope.com/jargon/d/defragme.htm and https://en.wikipedia.org/wiki/Disk_Defragmenter_(Windows)