Real Dictionary

water polo
uncountable noun: Water polo is a game played in a swimming pool in which two teams of swimmers try to score goals with a ball.

water polo in American English
a water game played with a round, partly inflated ball by two teams of seven swimmers, the object of the game being to pass or take the ball over the opponent's goal line
an aquatic game played by two teams of seven swimmers each, the object being to score goals by pushing, carrying, or passing an inflated ball and tossing it into the opponent's goal, defended by a goalkeeper

water polo in British English
noun: a game played in water by two teams of seven swimmers in which each side tries to throw or propel an inflated ball into the opponents' goal