In computer disk terminology, fragmentation is the process or condition of files that are composed of more than one extent. Fragmentation of this sort is bad, as it will increase the amount of head motion needed to read the file, and thus slow things down. Poorly optimized operating systems occasionally need to be defragmented to increase performance.

Alternately, in the BSD fast filesystem (also in UFS), a fragment is a block that has been subdivided to hold tail ends of several files that would otherwise each take up an entire block. This kind of fragmentation is a Good Thing.

Just to be confusing, both meanings of fragmentation are used when refering to unix filesystems.