Learn in the textbookFile System Basics — Directory Management and Backup
DefinitionFragmentation (フラグメンテーション, also 断片化) is the state in which the data of one file is recorded in separate places spread across a storage device.
ExplanationAs files are created and deleted over and over, free space breaks into small pieces, and new data can no longer be written in one continuous stretch. Reading split data means seeking out distant locations one after another, so reads take longer on a hard disk. The process that fixes this by rearranging scattered data into contiguous areas is called defragmentation (デフラグ).
AnalogyWhen a parking lot has only scattered empty spaces, a group arriving in several cars cannot park together, and collecting them later means driving all over the lot.
Memory hookA "fragment" is a broken-off piece. Picture a file shattered into pieces.