hard disk

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A rigid magnetic disk fixed permanently within a drive unit and used for storing computer data.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computing One of a series of parallel magnetic recording disks in a drive unit, used for the recording and retrieval of digital information; used especially in personal computers.
• noun computing The unit itself, and all the disks within it.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a rigid magnetic disk mounted permanently in a drive unit