The term 'MASTERMIND GAME PIECE' can be summarized as a three-letter word, 'PEG'. This word is used to refer to the small plastic pieces used in the classic board game Mastermind. The game involves two players, one of whom creates a secret code using the pegs, and the other player must guess the code by placing their own pegs on the board. The pegs are used to indicate the correct colors and positions of the secret code.