Lasting for, extending over, or remaining during a night: an overnight trip; an overnight guest. For use over a single night or for a short journey: overnight supplies. Mailed for guaranteed delivery on the next day: an overnight package. Happening as if in a single night; sudden: an overnight success. During or for the length of the night: Let the meat marinate overnight. In or as if in the course of one night; suddenly: became a sensation overnight. An overnight stay or trip. To spend the night: overnighting at a country inn. To send by mail or other courier for delivery the next day.