/dee-frag(-ment)'/ (Or "defrag") To coalesce files and free space on a file system.
As modern file systems are used and files are deleted and created, the total free space becomes split into smaller non-contiguous blocks (composed of "clusters" or "sectors" or some other unit of allocation). Eventually new files being created, and old files being extended, cannot be stored each in a single contiguous block but become scattered across the file system. This degrades performance as multiple seek operations are required to access a single fragmented file.
Defragmenting consolidates each existing file and the free space into a continuous group of sectors. Access speed will be improved due to reduced seeking.
The rate of fragmentation depends on the algorithm used to allocate space and the number and position of free sectors. A nearly-full file system will fragment more quickly.
msdos and microsoft windows use the simplest algorithm to allocate free clusters and so fragmentaton occurs quickly. A disk should be defragmented before fragmentation reaches 10%.