To defrag a computer is to tidy a hard disk so that pieces of the same file sit next to each other again. On the older kind of spinning hard drive, a file often ended up in little bits scattered around the disk, and the machine had to jump about to read them; putting those bits back in order could make the computer feel faster. People say defrag in everyday tech talk — it is the short, casual form of defragment. You defrag a disk or a drive, not a single file, and you do not “defrag” a messy desk: the word stays in the computer world. Though actually, phones and most new computers use storage with no spinning disk, and that kind of storage does not work the old way — you usually should not defrag it. The word itself feels a little dated, like something people did to a Windows PC in the 1990s and 2000s. Tech people still know it.

磁碟重組（台灣說法；中國大陸多說「磁盤碎片整理」）