Photodissociation (also called photolysis or photodecomposition) is a chemical reaction using the energy of an incoming photon to break a chemical bond, i.e., to dissociate two parts of the molecule. The higher the energy, the more likely the photon has the energy to carry out the dissociation, thus gamma rays, X-rays and ultraviolet rays can dissociate the largest number of compounds.
A photodissociation region (or photon-dominated region or PDR) is an interstellar gas cloud in which there are sufficient ultraviolet photons to dissociate a significant number of molecules. Such regions occur in at the border between molecular clouds and HII regions, in the region where far ultraviolet can penetrate.
Nuclear photodissociation is an unrelated nuclear reaction, essentially, fission triggered by a photon rather than by (for example) an incoming neutron. It can occur in some stellar processes and events.