Diurnal: Occurring in the daytime. A patient may have a diurnal fever rather than a nocturnal one. Diurnal also refers to something that recurs every day. QUESTION What causes tooth decay? See Answer From Healthy Resources 10 Rheumatoid Arthritis SymptomsWhat Is Exocrine Pancreatic Insufficiency?Truth About Vitamin B12 Featured Centers Good and Bad Foods for PsoriasisVideo: Getting Personal on Life With MS Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19