To reduce the magnetic flux to virtual zero by applying a reverse magnetizing field. Degaussing any current generation hard disk (including but not limited to IDE, EIDE, ATA, SCSI and Jaz) will render the drive permanently unusable since these drives store track location information on the hard drive.
Also called “demagnetizing.”
See NIST SP 800-88 Rev. 1 under Degauss for more information.