A process that occurs as a result of interactions between atomic nuclei when the interacting particles approach each other to within distances of the order of nuclear dimensions (~10⁻¹² cm). While nuclear reactions occur in nature, understanding of them and use of them as tools have taken place primarily in the controlled laboratory environment. In the usual experimental situation, nuclear reactions are initiated by bombarding one of the interacting particles, the stationary target nucleus, with nuclear projectiles of some type, and the reaction products and their behaviors are studied. The study of nuclear reactions is the largest area of nuclear and subnuclear (or particle) physics; the threshold for producing pions has historically been taken to be the energy boundary between the two fields.