"stimulation or inhibition to breeding, etc., based on daily periods of light and darkness," 1920, from photoperiod "period of daily illumination which an organism receives" (1920, from photo- + period) + -ism.
photoinduction
photojournalism
photometer
photomontage
photon
photoperiodism
photophobia
photoshop
photosphere
photostat
photosynthesis
)window.location='https://www.paypal.com/cgi-bin/webscr?cmd=_donations&business=byronic106@yahoo.com&lc=US&item_name=Donation+to +Help+Keep+Etymonline+Free+and+Open&no_note=0&cn=&curency_code=USD&bn=PP-DonationsBF:btn_donateCC_LG.gif:NonHosted'){kind=link}