Source : Free On-Line Dictionary of Computing

defragment
     
         /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%.
     
        (1997-08-29)