Dutch door

Wiktionary

1. n. 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

GNU Webster's 1913

1. adj. a door divided into two parts, horizontally, so arranged that the lower part can be shut and fastened, while the upper part remains open.

WordNet 3.0

1. n. an exterior door divided in two horizontally; either half can be closed or open independently