The Nuttall Encyclopedia
Sacred rites and ceremonies of stated observance among the Greeks and Romans in connection with the worship of particular divinities, to which only the initiated were admitted, and in which, by associating together, they quickened and confirmed each other in their faith and hope, and in which it would seem they made solemn avowal of these; the name is also applied to the miracle plays (q. v .) of the Middle Ages.
Wood, James, ed. Entry for 'Mysteries'. The Nuttall Encyclopedia. http://www.studylight.org/encyclopedias/nut/m/mysteries.html. Frederick Warne & Co Ltd. London. 1900.