Defragmentation is a process where data on computer hard drive is reorganized in order to speed up the HDD performance.
When files are stored to a hard drive, they aren't usually stored as a one "lump" to a particular place in a HDD, but instead, the file is split into several small chunks across the HDD. The file system then keeps record where each piece for the particular file is stored and seeks them from HDD when the file is accessed. But this seeking takes much more time that what it would take if the file would be stored in a continuous block. This is where defragmentation process comes into picture. Defragmentation process basically moves the small chunks that make up one file, into one block and therefor speeds up the HDD performance -- sometimes very significantly.
Keywords: hard drive defragmentation defrag speed up hdd reorganize data disk