defrag
Defrag is a process to rearrange files in a memory unit and solve the problem of fragmentation of free space, or a utility soft that performs defrag. It is an abbreviation of "defragmentation".
When files are written into and deleted from a disk again and again, the arrangement of files within a disk becomes discontinuous, and the free space becomes segmentalized. As segmentation goes and continuous free space decreases, more files that cannot fit into any free space increases, which are saved by being split into more than two free spaces.
When more files are saved by being split, it increases the amount of OS processing and drive head shifting, which makes the speed of reading and writing slower. Moreover, because the head shifting includes physical operation, it tends to induce a breakdown (the duration is shortened).
Defrag rearranges the files in a disk, solve the fragmentation of free space, and increase continuous free space. It had been sold as independent application software, but lately it often comes as a function of OS from the beginning.