What is Mystery?

Legal Definition
A trade, art or occupation. 2 Inst. 668. Masters frequently bind themselves in the indentures with their apprentices to teach them their art, trade, and mystery. State v. Bishop, 15 Me. 122; Barger v. Caldwell, 2 Dana (Ky.) 13L
-- Black's Law Dictionary
Legal Definition
An occupation, business or calling. See 15 Me. 122.
-- Ballentine's Law Dictionary