1. Of a thing or image, strikingly clear or clean. (Sometimes hyphenated when used as a modifier before a noun.) The picture on this new high-definition TV is crystal clear!The crystal-clear skies at the top of the mountain afforded a spectacular view of the whole state down below.
2. Of information or communication, very easy to understand; not vague or ambiguous. (Sometimes hyphenated when used as a modifier before a noun.) A: "You have to drop this package off by 5 PM sharp, or it won't get delivered. Is that clear?" B: "Crystal clear."A good lecturer provides crystal-clear lessons in class, but a great one makes them engaging.