The Collaborative International Dictionary of English v.0.48:

Anteroom \An"te*room\, n.
   A room before, or forming an entrance to, another; a waiting
   room.
   [1913 Webster]

WordNet (r) 3.0 (2006):

anteroom
    n 1: a large entrance or reception room or area [syn:
         anteroom, antechamber, entrance hall, hall,
         foyer, lobby, vestibule]