In the game of chess, prophylaxis (Greek προφυλαξις, "prophylaxis," guarding or preventing beforehand) or a prophylactic move is a move that stops the opponent from taking action in a certain area for fear of some type of reprisal. Prophylactic moves are aimed at not just improving one's position, but preventing the opponent from improving their own. Perhaps the most common prophylactic idea is the advance of the rook pawn near a castled king to make luft averting the possibility of a back rank checkmate, or to prevent pins.
In a more strategic sense, prophylaxis leads to a very positional game, often frustrating for players with a strong tactical orientation. Players who play in the prophylactic style prevent the initiation of tactical play by threatening unpleasant consequences. One of the largest advantages of this approach is that it keeps risk to a minimum while causing an overaggressive opponent to lose patience and make a mistake. The disadvantage is that it frequently fails against an opponent who is content with a draw.
It should be understood, however, that any move which prevents an opponent from threatening something can be called prophylactic, even if this word would not be used to describe the player's style. For example, Mikhail Tal and Garry Kasparov frequently played the move h3 in the Ruy Lopez - a prophylactic move intended to prevent Black from playing ...Bg4 and creating an irritating pin on the knight at f3 - yet neither player would ever be described as playing in the prophylactic style. All grandmasters make use of prophylaxis in one way or another.
Famous practitioners include: