A lock file that stayed around even though it should have been deleted (e.g. the associated resource is no longer in use).

Stale locks typically occur when a program (or the computer) crashes after a lock file was created. One common strategy for avoiding stale locks (at least in Unix) is to put the PID of the creating process into the file. If the process designated by the file no longer exists, the lock file may be considered invalid (as the process obviously neglected to delete it before it terminated).
