pho·to·pe·ri·od·ism \ˌfōd.(ˌ)ōˈpirēəˌdizəm\noun alsopho·to·periodicity\ˌfōd.(ˌ)ō+\ (pluralphotoperiodismsalsophotoperiodicities) Etymology:photoperiodism International Scientific Vocabulary phot- + period + -ism; photoperiodicity from photoperiodic (from phot- + periodic) + -ity : capacity to respond to the photoperiod