diurnalis

Diurnālis comes from diurnus (of the day), from dies (day). The adjective describes anything that happens during the daytime or has a daily cycle. Diurnal animals are active during the day. Diurnal temperature variation is the difference between day and night temperatures. The word was scientific Latin from the start — a classification term, not a poetic one.

The same Latin dies produced an enormous family of English words. Through French jour (from Vulgar Latin *diurnum), English got 'journal' (a daily record), 'journey' (originally a day's travel), 'sojourn' (to spend the day), and 'adjourn' (to put off to another day). Through the Latin adjective quotidianus (of each day), English got 'quotidian' (daily, ordinary). All of these are about days. None of them sound alike.

In biology, 'diurnal' is the technical opposite of 'nocturnal.' Humans are diurnal. Owls are nocturnal. Cats are crepuscular (active at dawn and dusk). These classifications describe an animal's activity patterns in relation to the light cycle. The words are Latin, but the biology is universal — every organism on Earth evolved its activity cycle in response to the same sun.

The word also has an astronomical sense: the 'diurnal motion' of the stars is their apparent movement across the sky caused by the Earth's rotation. One diurnal cycle equals one rotation of the Earth. This is the original and most literal meaning — the daily cycle, the day's motion, the thing that happens once per dies.