A tennis ball is a ball

Ball

A ball is a round, usually spherical but sometimes ovoid, object with various uses. It is used in ball games, where the play of the game follows the state of the ball as it is hit, kicked or thrown by players. Balls can also be used for simpler activities, such as catch, marbles and juggling...

 designed for the sport of tennis,
approximately 6.7 cm (2.63 in.) in diameter. Tennis balls are generally bright green, but in recreational play can be virtually any color. Tennis balls are covered in a fibrous fluffy felt

Felt

Felt is a non-woven cloth that is produced by matting, condensing and pressing woollen fibres. While some types of felt are very soft, some are tough enough to form construction materials. Felt can be of any colour, and made into any shape or size....

 which modifies their aerodynamic properties. Tennis balls are usually green at main sporting events.
Before the development of lawn tennis in the early 1870s, the sport was played as the courtly game of real tennis

Real tennis

Real tennis – one of several games sometimes called "the sport of kings" – is the original indoor racquet sport from which the modern game of lawn tennis , is descended...

.