leave of absence

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

• noun Official permission to be absent from work or duty, as that granted to military or corporate personnel.
• noun The period of time granted by such permission.

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

• noun A period of time during which a person is absent from work or other duty for a reason other than normal holidays / vacation; may be paid or unpaid.

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

• noun the period of time during which you are absent from work or duty