The slack

A file is saved in chunks of space called clusters. The size of a cluster depends on the size of a drive partition. As the operating system always allocates a whole number of clusters to a file, the last cluster of the file may contain some unused space (unless the size of the file is an even multiple of the cluster size). This unused space is called file slack. The file slack is filled with random information that comes from your computer. It may contain sensitive data from the file(s) that previously occupied the cluster or from your computer memory.

The slack