Crepuscular is a term used of animals to indicate they are active during the twilight. Crepuscular is thus in contrast with diurnal and nocturnal. Many animals that are casually described as nocturnal are in fact crepuscular, though animals that are crepuscular may be active on a bright moonlit night. Within the definition of crepuscular are the terms matinine and vespertine, denoting animals active in the morning and evening respectively.
Crepuscular mammals include the common mouse and rat. Crepuscular birds include Common Nighthawk and Spotted Crake. Some species have different habits in the absence of human or other predators. Thus the Short-eared Owl is diurnal on those of the Galapagos islands that do not have buzzard species, but crepuscular on the others.