Data cluster

A data cluster (or cluster or file allocation unit) is an elementary unit of allocation of storage media, such as a floppy disk or hard disk made up of physical blocks that are indexed by a File Allocation Table (FAT).

Description
A file is made up of a whole number of one or more clusters. The clusters may not necessarily be stored contiguously on the drive, a state known as fragmentation. The cluster size is a tradeoff between space efficiency (the larger the cluster size, the bigger on average is the wasted unused space past the end of the file in the last cluster), and the size of the File Allocation Table. A failing drive can lead to lost clusters, where segments of data become corrupted or missing.