Storage of data in non-contiguous areas on disk. As files are updated, new data is stored in available free space, which may not be contiguous. Fragmented files cause extra read/write head movement, slowing disk accesses.
The on-going process on an active file system in which the file system is spread further and further along the disk, leaving unused gaps or fragments between areas that are in use. This leads to degraded performance because the file system has fewer options when assigning a file to an extent.