Daily (3,4)
(Other definitions for per diem that I've seen before include "living expenses" , "For each day (Latin)" , "By the day, to the old Romans" .)
'daily' is the definition.
(associated in meaning)
This is all the clue.
(Other definitions for diurnal that I've seen before include "Happening daily" , "in daylight" , "every day" , "Daily; during the daytime" , "Active during the day" .)