A lost cluster is any unit of storage space (a cluster) on a MS-DOS/Windows-formatted hard drive that is not used by a file, and is not listed as free.
Lost clusters can contain parts of corrupted files, but they are normally just blank space that was allocated to a program, but was not unallocated by that program.
Lost clusters are sometimes created when PCs are reset or turned off without properly closing each of the processes running at the time. This includes Windows itself.
Lost clusters can be detected using a variety of file system diagnostic utilities, including CHKDSK (for DOS 5 and below), SCANDISK (for DOS 6 and Windows 95) and Norton Disk Doctor (for DOS 6.2 and below).
If lost clusters are detected, they can usually be safely deleted by the program that finds them. The contents of lost clusters can be recovered, but the resultant files are normally just garbage. Until they are repaired, lost clusters consume space and may cause further corruption.
To minimise the chances of developing lost clusters, close all programs before resetting or powering off. In Windows 95, this means using the shutdown command; in Windows 3.1, this means exiting Windows via Program Manager.