Defragment

Fragmentation is a condition in which pieces of individual files on a disk volume are not contiguous, but rather are broken up and scattered around the disk.

Fragmentation also refers to a condition in which the free space on a disk volume consists of little pieces of space here and there, rather than a few large free spaces.

Thus to defragment a disk volume is to make individual files on the volume contiguous, and also to consolidate the free space on the volume.