1) Of a 24-hour period 
pertaining to the day-night cycle. 
Term frequently used synonymously with diurnal or circadian. ... Origin: irreg., fr. L. Dies, day ... (05 Mar 2000) ... 
Type: Term Pronunciation: dī′el Definitions: 1. Term frequently used synonymously with diurnal (2) or circadian.
of or pertaining to a 24-hour period, esp. a regular daily cycle, as of the physiology or behavior of an organism.