A utility that rearranges files and directories on a disk for optimum performance. By reducing or eliminating file fragmentation (storage in pieces in different locations on the hard disk), a disk optimizer can restore the original level of performance of your disk system. Also, it is usually easier to undelete, or recover, an unfragmented file than a fragmented one.
Many disk optimizers will not only rewrite files as contiguous files, but will also place specific unchanging files in particular locations on the disk, optimize directories, and even place specific applications on the disk so that they load more quickly.
See also defragmentation.