An extent is a contiguous area of storage in a computer file system, reserved for a file. When a process creates a file, file-system management software allocates a whole extent. When writing to the file again, possibly after doing other write operations, the data continues where the previous write left off. This reduces or eliminates file fragmentation and possibly file scattering too.
An extent-based file system (i.e., one that addresses storage via extents rather than in single blocks) need not require limiting each file to a single, contiguous extent.
The following systems support extents:
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The CP/M file system uses extents as well, but those don't correspond to the definition given above. CP/M's extents appear contiguously as a single block in the combined directory/allocation table; they do not necessarily correspond to a contiguous data-area on disk.