A Chabadnitze (Yiddish: חב"דניצע, also known as a חדר שני (Cheder sheini) "second room" in Hebrew) is a small side room required for a Chabadsynagogue. It is designated specifically for those who wish to pray at length, in the Chabad tradition of meditation, in a quiet setting. Accordingly, the sixth Lubavitcher Rebbe, Rabbi Yosef Yitzchok Schneersohn related:
In years gone by they would build an antechamber in every shul that they called a “Chabadnitze.” In the time of the Mittele Rebbe there was a minyan in the town of Makolyene near Lubavitch that didn’t have a Chabadnitze. The Mittele Rebbe said they couldn’t daven there; you could not say K’dusha and Barchu in a minyan without a Chabadnitze. Whether they prayed at length or not, there had to be a Chabadnitze!