1 (used about animals and birds) awake and active at night and asleep during the day （指动物及鸟类）白天睡觉而夜间活动的: Owls are nocturnal birds. 猫头鹰是昼伏夜出的鸟类。 [OPP] diurnal 反义词为diurnal

2 (written 书面语) happening in the night 夜间发生的: a nocturnal adventure 夜间冒险行动