Alternatively referred to as disk compression, a disk optimizer is software designed to organize or compress data to maximize free disk space.
A good example of a disk compression software program is a Microsoft Windows utility known as double space.
With today's large hard drives, disk compression utilities are no longer used and no longer recommended. A disk compression utility should be used on hard drives of 1 gig and smaller.
The Microsoft Windows utility defrag can be considered a disk optimizer and is still commonly used and recommended even on large hard drives.