Prophylaxis (chess)

In the game of chess, prophylaxis (Greek προφύλαξις, "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 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 , 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.
Any move that 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. 
Advanced prophylactic play cannot usually be employed by novice players. However, many standard and widespread opening moves can be considered prophylactic. 
The board above shows a common tactical position. White is looking for Nf7, resulting in a fork on the queen and rook, guaranteeing a material advantage - as the king cannot take the knight as it is defended by the bishop. In response, black plays h6, denying the knight of the g5 square, thus anticipating the attack.
Pawn moves such as h6 or a6 don't require such an immediate threat in order to be played. They can often be played as recreational moves to stop tactics of bishops or knights occupying the g5/b5 squares entirely. Bishops have incentive to be played on these squares as it results in pins on the queen and king respectively - forcing the knight to stay stationary. 
Famous practitioners include: