Real Dictionary

tennis
uncountable noun: Tennis is a game played by two or four players on a rectangular court. The players use an oval racket with strings across it to hit a ball over a net across the middle of the court.

tennis in American English
a game, usually played outdoors, in which two players or two pairs of players using rackets hit a fabric-covered, hollow rubber ball back and forth over a net stretched across a marked, level, rectangular area (tennis court)
noun: a game played on a rectangular court by two players or two pairs of players equipped with rackets, in which a ball is driven back and forth over a low net that divides the court in half

tennis in British English
noun: a racket game played between two players or pairs of players who hit a ball to and fro over a net on a rectangular court of grass, asphalt, clay, etc