A portion of a disk that cannot be used because it is flawed. When you format a disk, the operating system identifies any bad sectors on the disk and marks them so they will not be used. If a sector that already contains data becomes damaged, you will need special software to recover the data.
It’s not unusual for a hard disk to leave the manufacturing process with a damaged sector; however, this does not affect the overall performance of the disk as the damaged space is considered unusable.
Hard Disk Interfaces and Configuration Describes the different major interface standards currently used by hard disks (and other devices). Provides sections about the IDE/ATA and SCSI interfaces. This page is from "The PC Guide."