File fragmentation means that a file

Concept

File fragmentation is a condition that arises in secondary storage (the disk). A file is normally written into a set of storage blocks; when there is no single run of free blocks large enough to hold the whole file, the file system splits it and places its pieces in whatever scattered free blocks are available. The file is then said to be fragmented: its data occupies non-contiguous (non-adjacent) locations on the disk rather than one continuous region.

Application

By that definition, saying a file is fragmented means its blocks are stored in non-contiguous locations on the disk. The data is still complete and readable; only its physical placement is scattered, which is why fragmentation tends to slow access (the read head or controller must jump between separated regions). This matches the statement that the file is stored in non-contiguous locations on disk.

Contrast

• Non-contiguous locations in RAM: fragmentation in this question refers to on-disk storage (a permanent, file-system property). RAM fragmentation is a separate memory-management idea and is not what 'file fragmentation' denotes.

• Has been compressed: compression reduces a file's size by re-encoding its data; it is unrelated to whether the file's blocks are adjacent on disk.

• Unusable until defragmented: a fragmented file remains fully usable and readable; defragmentation only reorganises the blocks to be contiguous for better performance, it does not restore lost access.