Dutch door

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

• noun A door divided in two horizontally so that either part can be left open or closed.

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

• noun a door that is divided into two horizontally such that either part (or usually just the upper part) may be opened independently of the other

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

• noun an exterior door divided in two horizontally; either half can be closed or open independently