Mith·rae·um or mith·rae·um 
(mĭthrē-əm)
(mĭthrē-əm)n. pl. Mith·rae·a (-rē-ə) or mith·rae·a
An often subterranean or semisubterranean temple dedicated to the worship of Mithras.
[New Latin, from Greek Mithraion, from Mithrās, Mithras; see MITHRAS.]