Zeitgeber (from German for "time giver," or "synchronizer") is any exogenous (external) cue that synchronizes an organism's endogenous (internal) time-keeping system (clock) to the earth's 24-hour light/dark cycle. The strongest zeitgeber, for both plants and animals, is light. Non-photic zeitgebers include temperature, social interactions, pharmacological manipulation, exercise, and eating/drinking patterns. To maintain clock-environment synchrony, zeitgebers induce changes in the concentrations of the molecular components of the clock to levels consistent with the appropriate stage in the 24-hour cycle, a process termed entrainment.
The German term zeitgeber came into the English language when Jürgen Aschoff, one of the founders of the field of chronobiology, used it in the 1960s. It is now in common use in the scientific literature in this field.
Information about "Zeitgeber" gathered from wikipedia.
