a chamber or room that serves as a waiting room and entrance to a larger room or an apartment; anteroom.

Why is it called a antechamber?

antechamber (n.)

"chamber which gives access to a principal chamber; waiting room," 1650s, antichamber, from French antichambre (16c.), on analogy of Italian anticamera (see ante- and chamber (n.)).

Is antechamber a root word?

Word Origin. (as antichamber): from French antichambre, from Italian anticamera, from anti- 'preceding' + camera 'vault, arched chamber', from Greek kamara 'object with an arched cover'.