adv. phr. 1. At the same time; together. The teacher told the children to talk one at a time; if they all talked at one time, she could not understand them. Bill can play the piano, sing, and lead his orchestra all at once. 2. or all of a sudden Without warning; abruptly; suddenly; unexpectedly. All at once we heard a shot and the soldier fell to the ground. All of a sudden the ship struck a rock.
Compare: AT ONCE.