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An external stimulus or cue, such as daylight or a regularly repeated occurrence, that serves to regulate an organism's biological clock.
[German Zeitgeber : Zeit, time; see Zeitgeist + Geber, giver (from geben, to give, from Middle High German, from Old High German geban; see ghabh- in Indo-European roots).]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Biology) biology an agent or event, such as light or the tide, that triggers the biological clock of organisms
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
an environmental cue, as the length of daylight or the degree of temperature, that helps to regulate the cycles of an organism's biological clock.
[1970–75; < German (1954), literally, time-giver]
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.