A type of data compression that works by storing compressed versions of files on the hard disk. A disk compression utility sits between the operating system and the disk drive. Whenever the operating system attempts to save a file to disk, the utility intercepts it and compresses it. Likewise, when the operating system attempts to open a file, the disk compression utility intercepts the file, decompresses it, and then passes it to the operating system. Because all applications access files through the operating system, disk compression utilities work with all applications. The entire process is transparent to the user, though opening and closing files may take a little longer. On the other hand, a disk compression utility can double the amount of disk space available.