boot block

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

• noun An area of a disk having information for loading the operating system that is needed to start a computer.

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

• noun computing A dedicated block usually at the beginning (first block on first track) of a storage medium that holds special data used to start a system. Some systems use a boot block of several physical sectors, while some use only one boot sector. Other manufacturers use the terms boot block and boot sector interchangeably.