Coefficient of restitution, e= relative speed of two objects after impact divided by their relative speed before impact; or the ratio of these speeds.
When e=1, the collision is perfectly elastic, and when e=0, the collision is perfectly inelastic (the objects coalesce or the kinetic energy is completely dissipated).
A typical example illustrating e is a bouncing ball on solid ground: the diminishing height of each bounce is a measure of the coefficient for the ball and the surface on which it is bouncing.
Sorry, I don't have the dictionary definitions either.