Definition of REGENERATION. 1. : an act or the process of regenerating: the state of being regenerated. 2.: spiritual renewal or revival. 3. : renewal or restoration of a body, bodily part, or biological system (as a forest) after injury or as a normal process. 4.efinition of REGENERATION. 1. : an act or the process of regenerating: the state of being regenerated. 2. : spiritual renewal or revival. 3.