Literally translates to "time giver". It refers to the stimulus that entrains biological rhythms. Light is a powerful example. 

Zeitgeber

The process of synchronizing a biological rhythm to an environmental stimulus. 

- Zeitgeber

University of Rochester

The endogenous clock can be adjusted through a variety of means, though the most common is light. 
- Phase shift
- Entrainment 


Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

A shift in the activity of a biological rhythm, typically provided by a synchronizing environmental stimulus. 

- Zeitgeber