Diel

Diel means "in the course of the day". Thus a "diel variation" is a variation that occurs regularly every day or most days.

In many sources, the word diurnal is used in the same sense. However, in more modern or more technical usage, diurnal is reserved for the meaning "during the daytime", i.e. in contrast with nocturnal (during the nighttime) or crepuscular (during the twilight). "Diel" is therefore to be preferred where its use would not be pedantic.