Zeitgeber: An environmental agent or event that provides the cue for setting or resetting a biological clock. To be synchronized with our environment, we need the input of Zeitgebers. The most important Zeitgeber in nature is light. Social factors, chemical factors and activity can also serve as Zeitgebers. Light is a so-called photic type Zeitgeber, whereas activity, for example, is called a non-photic Zeitgeber for the biological clock.
Zeitgeber comes from the German Zeit, time + Geber, giver = time-giver. The term Zeitgeber was introduced into science about 1954 by Jurgen Aschoff, then the Director of the Max-Planck-Institute in Erling-Andechs, near Munich.