length of daylight and/or darkness required for certain developmental processes and growth in plants.
Merriam-Webster Online Dictionary
photoperiod (noun) a recurring cycle of light and dark periods of constant length , also - photophase
photoperiod (Wikipedia)
Photoperiodism is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals. Photoperiodism can also be defined as the developmental responses of plants to the relative lengths of light and dark periods. They are classified under three groups according to the photoperiods: short-day plants, long-day plants, and day-neutral plants.