Fragment is the term used in the ExtX and UFS file systems to refer to a grouping of consecutive sectors. Sectors are typically 512-bytes each and it is more efficient for a file system to allocate and use multiple sectors at a time. Each file system uses a different term for the resulting group. TSK uses the term data unit to refer to the grouping.
These file systems will also group consecutive fragments into blocks, but block addresses are equal to the fragment addresses.
Note that while ExtX file systems support fragments and blocks, they typically are equal sizes (meaning that 1 block is equal to 1 fragment).