1. The term diurnal means active in the daytime. This definition is often related to animals or plants. The opposite of the nocturnal or crepuscular.
2. Diurnal means daily, especially pertaining to actions which are completed in 24 hours and are repeated every 24 hours. In meteorology this can be seen in the diurnal temperature variation. It also applies to the stars as in diurnal motion. An alternative word for "diurnal" in this sense is diel