Emily Dickinson Lexicon

DI-URN'AL, a. [L. diurnus, daily; W. diwrnod, a day. The word is a compound of diw, dies, day, and a word which I do not understand.]

1. Relating to a day; pertaining to the day-time; as, diurnal heat; diurnal hours.
2. Daily; happening every day; performed in a day; as, a diurnal task.
3. Performed in 24 hours; as, the diurnal revolution of the earth.
4. In medicine, an epithet of diseases whose exacerbations are in the day-time; as, a diurnal fever. – Parr.