Main Entry: pho·to·in·ac·ti·va·tion Pronunciation: "fOt-O-(")in-"ak-t&-'vA-sh&n Function: noun 1: the retardation or prevention of a chemical reaction by radiant energy (as light) —compare PHOTOCATALYSIS 2: the inactivation of a substance by radiant energy (as light) —compare PHOTOACTIVATION