Photoperiodism is a term in biology for the way a plant or animal reacts to the amount of light it gets at a time, including flowering or not flowering.
An example of photoperiodism is when a plant doesn't bloom during the increased darkness of winter time.
Biol. the behavioral or physiological reaction of an organism to variations in the duration of light, as, in plants, by flowering or ceasing to flower, or, in certain animals, by changing the daily cycle of activities
also photoperiodicity (fōˌ◌tō-pîrˌ◌ē-ə-dĭsˌ◌ĭ-tē)
The response of an organism to changes in its photoperiod, especially as indicated by vital processes. For example, many plants exhibit photoperiodism by flowering only after being exposed to a set amount of daylight, as by requiring either a long or short day to flower. Plant growth, seed germination, and fruiting are also affected by day length. Photoperiodic responses in plants are regulated by special pigments known as phytochromes. In animals, migration, mating, amount of sleep, and other behaviors are also photoperiodic. In many animals, photoperiodism is regulated by the hormone melatonin.