propylæum

from The Century Dictionary.

• noun An important architectural vestibule or entrance to a sacred inclosure or other precinct, as that of the Acropolis of Athens, or that of the sanctuary of Eleusis: usually in the plural.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Anc. Classical Arch.) Any court or vestibule before a building or leading into any inclosure.