Most table tennis balls used celluloid before plastic balls became standard.
Celluloid balls dominated table tennis for more than a century because the material was light, resilient, and capable of producing the familiar bounce. Early table tennis equipment developed in the late nineteenth and early twentieth centuries, and celluloid became closely associated with the sport’s traditional white or orange ball.
The International Table Tennis Federation approved non-celluloid plastic balls for international play during the 2010s. The change was driven partly by celluloid’s flammability and manufacturing restrictions. Plastic balls gradually replaced the older type in major competitions, although players often noticed differences in sound, bounce, and spin.
“Plastic ball” describes the newer category, not one single chemical formula. Modern balls can be made from several plastics, so celluloid is the specific historical material that the question asks about.