ˈtsītˌgābər, ˈzīt- noun
Etymology: German, from zeit time (from OGH zīt ) + geber (as in taktgeber computer clock), literally, giver, from geben to give, from Old High German geban ; akin to Old English giefan to give — more at tide give
: an environmental agent or event (as the occurrence of light or dark) that provides the stimulus setting or resetting a biological clock of an organism