A shiner is any small reflective object a cheat uses to glimpse cards as they are dealt, such as a tiny mirror hidden in the hand, a polished lighter or a ring. Positioned under the dealer’s hand, it lets the cheat see the face of the dealer’s hole card or the next card from the deck, information that changes how a hand should be played.
Using a shiner is cheating. A player caught with one can expect to be removed from the casino and lose any winnings and, depending on local law, may face prosecution. Casinos counter the threat through dealing procedures and the surveillance of the eye in the sky; in live dealer studios, controlled card handling and camera angles serve the same purpose.