A variation on carom billards in which the player must contact both object balls, but must hit three rails, or cushions, in between hitting the two balls.
Thus, a player strikes the cue ball at one object ball. After it hits it, it must hit three rails (though it may hit more), and then strike the second object ball. If the play is successful, he or she scores a point. If not, then their inning is over and the other player steps up.
Generally, in tournament play, the players play to a certain agreed upon score, that can range from 500 to 2000. Watching people play three cushion gives you an aesthetic pleasure towards practical geometry unmatched by the finest architecture in the world.