Slack Space - Definition

Slack space is a form of internal fragmentation, i.e. wasted space, on a hard disk. When a file is written to disk it’s stored at the “beginning” of the cluster. A cluster is defined as a collection of logically contiguous sectors and the smallest amount of disk space that can be allocated to hold a file. Rarely will there be an even match between the space available in a cluster (or collection of clusters for longer files) and the number of bytes in the file. Left over bytes in the cluster are unused, hence the name slack space.