An outer room that leads to another and is often used as a waiting room.
Example:
The antechamber to the lawyer's office was both elegant and comfortable, designed to inspire trust and confidence.
Antechamber suggests a room somewhat more formal than an anteroom. One expects to find an antechamber outside the private chambers of a Supreme Court Justice or leading into the great hall of a medieval castle. In the private end of the castle the lord's or lady's bedchamber would have its own antechamber, which served as a dressing room and sitting room, but could also house bodyguards if the castle came under siege.