ad. One time only: once a day. ad. At one time in the past; formerly.ad. At any time; ever: Once known, his face is never forgotten. ad. By one degree of relationship: my first cousin once removed. n. A single occurrence; one time: Once will have to do. You can go just this once. conjunction. As soon as; if ever; when: Once he goes, we can clean up. adj. Having been formerly; former: the once capital of the nation. idiom. at once All at one time; simultaneously: Everything happened at once. The view of the skyline is at once awesome, grand, and disappointing. idiom. at once Immediately; instantly: Leave the room at once.