The Collaborative International Dictionary of English v.0.48:

Propylaeum \Prop`y*l[ae]"um\, n.; pl. Propyl[ae]a. [L., fr.
   Gr. ?; ? before + ? a gate.] (Anc. Classical Arch.)
   Any court or vestibule before a building or leading into any
   inclosure.
   [1913 Webster]