The meaning of NOCTURNAL is of, relating to, or occurring in the night . How to use nocturnal in a sentence. Nocturnal definition : of or relating to the night (diurnal ).. See examples of NOCTURNAL used in a sentence. Discover the meaning of ' nocturnal ', explore its usage in everyday language, popular idioms, and fun facts that make this word fascinating. Perfect for learners and curious minds! Explore the nocturnal definition , etymology, and significance in biodiversity. Discover facts about night -active species.