PHOTOPERIOD Meaning of PHOTOPERIOD in English [pho.to.pe.ri.od] n (1920): a recurring cycle of light and dark periods of constant length; also: photophase 2. -- pho.to.pe.ri.od.ic adj -- pho.to.pe.ri.od.i.cal.ly adv Merriam-Webster English vocab. Английский словарь Merriam Webster. 2012