An enigmatic word or phrase that appears, unexplained and without context, here and there throughout an Arc, and (with luck) is finally explained at or near the climax. A way of building up tension and mystery, as well as an indicator that anyone using the words knows more than they're telling. Can also be used as a memetic way of advertising the show. A typical element of a Mind Screw.
Arc Words [...] [tvtropes.org]