mystērĭum (noun N) = μυστήριον:
* A secret service, secret rites, secret worship of a deity, divine mystery (class.; cf. arcanum).
* Lit., of the mysteries of Ceres, otherwise called sacra Eleusinia, Cic. N. D. 2, 24, 62; id. Leg. 2, 14, 35: mysteria Attica,Tert. Apol. 39: mysteria Cereris initiorum enuntiare,Just. 5, 1, 1: mysteria facere,to celebrate the sacred mysteries,Nep. Alcib. 3, 6.—Also, the festival on which these mysteries were celebrated: in quem diem Romana incidant mysteria, the festival of the goddess Bona Dea, Cic. Att. 6, 1, 26; 5, 21, 14 sq.
* Transf., in gen., a secret thing, secret, mystery: rhetorum mysteria,Cic. Tusc. 4, 25, 55; id. de Or. 1, 47, 206: epistolae nostrae tantum habent mysteriorum,id. Att. 4, 18, 1: accipe congestas, mysteria frivolas nugas,Aus. Ep. 4, 67.
* (Eccl. Lat.)
* Something transcending mere human intelligence: mysterium evangelii,Vulg. Eph. 6, 19: mysterium sicut evangelizaverat per prophetas,id. Apoc. 10, 7: mysteria regni caelorum,id. Matt. 13, 11.
* Of Antichrist, Vulg. Apoc. 17, 5: mysterium iniquitatis,id. 2 Thess. 2, 7.
* The Lord's supper: mysterium celebrat,Ambros. in 1 Cor. 11, 27.

s. TLL