photoperiod

n.
The duration of an organism's daily exposure to light, considered especially with regard to the effect of the exposure on growth and development.

photoperiodic pho'to·pe'ri·od'ic (-ŏd'ĭk) or pho'to·pe'ri·od'i·cal (-ĭ-kəl) adj.

(fō'tō-pĭr'ē-əd)
n.

The duration of an organism's daily exposure to light, considered especially with regard to the effect of such exposure on growth and development.

pho'to·pe'ri·od'ic (-ŏd'ĭk) or pho'to·pe'ri·od'i·cal (-ĭ-kəl) adj.

The period of time per day that an organism is exposed to daylight (or to artificial light).

See daylength.