In chess, a "theoretical novelty" (often just called a "novelty") is the first move of a game that has never been played before in that has never been played before in that same position by any other player in the recorded history of chess. The term is most often used when such a completely unexpected move is played very early on in the game.
Because chess is such an old game, and has been around for thousands of years, at high levels of play, top players often play dozens of moves at the outset of the game that have been played before and intensively studied. These are "known lines" of chess theory with well-known responses. It is not until one of the players finally plays a novelty that the game really begins.
In chess notation, novelties are often marked with a single exclamation point (!) after the move notation.