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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.
(Biology) biology an agent or event, such as light or the tide, that triggers the biological clock of organisms
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]