Clobber is a combinatorial game for two players. It can be played on any graph, although usually it is played on a rectangular board (of any size) divided into squares (for example, a chess board or a go board).

The two sides, represented by white and black, have only one type of piece each, called a stone or pawn. Each node on the graph or square on the board can hold at most one piece. A legal move is to "clobber" one of your opponent's pieces by moving one square in any direction (possibly including diagonal, depending on the variant you are playing) onto a piece of the opposite color.

These are the only legal moves; one cannot move without clobbering, nor can one clobber ones' own pieces. A player called upon to move who has no legal move loses the game, as in most combinatorial game theory.

If one player has a possible move in a component of a clobber position, the other player does as well, so that one cannot store up territory, as in a game like Go or Domineering. Consequently, every clobber position has infinitesimal value, meaning the combinatorial value of a clobber position is between -1/n and 1/n for every integer n. Clobber positions may take nimber values like star or star two, or partizan values such as up, double up, or (maybe) up second.