In chemical terms, actinism is the property of radiation that lets it be absorbed by a molecule and cause a photochemical reaction as a result. Albert Einstein was the first to correctly theorize that each photon would be able to cause only one molecular reaction. This distinction separates photochemical reactions from exothermic reduction reactions triggered by radiation.
For general purposes, photochemistry is the commonly used vernacular rather than actinic or actino-chemistry, which are again more commonly seen used for photography or imaging.