telesterion
A building used to celebrate sacred rites in Ancient Greece.
Central among them is what, to avoid controversy, it is convenient to call the telesterion: a building designed to accommodate the faithful inside, and thus unique among the forms of Greek sacred architecture.
Where it comes from
From Ancient Greek τελεστήριον (telestḗrion, “initiation hall”), from τελείω (teleíō, “to initiate”).
Adapted from the Wiktionary entry telesterion (CC BY-SA 4.0).