A Stopper is a decoy letter that's usually seen on the game show Scrabble which in turn was based on the world-famous classic board game of the same name.
Usually seen in the Crossword round, if one of the two contestants found one of the "stoppers", then control along with any used letters were passed on to their opponent. However, if all three "stoppers" were found, then the remaining letters in the word were placed one at a time until someone buzzed in and solved the word. The "stoppers" would usually explode and appear as small-looking red stop signs.
Scrabble (1984-1990 & 1993)