DIURNAL, adjective [Latin , daily.] 1. Relating to a day; pertaining to the daytime; 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. DIURNAL, noun A day-book; a journal. [See Journal, which is mostly used.]
Source: Webster's 1828 American Dictionary ↗ · public-domain · returned verbatim, no AI