1. Active during the sunrise or sunset evening in the twilight hours: "The crepuscular activities of the bats flying out of the cave was a signal that a lot of bugs would be consumed very soon."
2. In zoology, active or primarily active at the time around dawn or dusk (evening); such as, rabbits, deer, bats, and other mammals, as well as various insects: "Rabbits are crepuscular animals, much like deer; so, during the day, they like to sleep in their burrows, in depressions of grass, or even in their cages."