Definition - What does Cluster (Disk) mean?
A cluster, in the context of a hard disk, is a group of sectors within a disk and is the grouping by which disk files are organized. A cluster is larger than a sector, and most files fill many clusters of disk space. The hard drive is able to find all the clusters on a disk because each cluster possesses its own ID.
Techopedia explains Cluster (Disk)
By using the disk utility program on a Mac or the ScanDisk program on Windows, users can free up disk space. This is known as defragmentation, and it boosts computer performance. Even if a user deletes several small files and adds a newer, larger file, that file may actually be contained in several small files within the hard drive. When the computer starts making unusual sounds, this may be a clue that clusters of small files need to be defragmented.