How would I describe the bounciness of a ball, not using that word? (it doesn't sound formal)
Kinetic elasticity refers the ability of a material to absorb kinetic energy and deform, and then return to its original shape releasing (some or most of) the energy again. A material with high kinetic elasticity (e.g. rubber) will be more "bouncy" because it will conserve more energy in a collision. Kinetic elasticity can also be used to describe the bounciness of a spring.
The most distinguishing characteristic of rubber is kinetic elasticity