What does "once" mean?

Additional senses

1. 2.Trees that bear mast are fruitful but once in two years. Bacon.
2. 3.At some one period of time; -- used indefinitely. My soul had once some foolish fondness for thee. Addison. That court which we shall once govern. Bp. Hall.
3. 4.At any one time; -- often nearly equivalent to ever, if ever, or whenever; as, once kindled, it may not be quenched. Wilt thou not be made clean When shall it once be Jer. xiii.
4. 5.To be once in doubt Is once to be resolved. Shak. Note: Once is used as a noun when preceded by this or that; as, this once, that once. It is also sometimes used elliptically, like an adjective, for once-existing. "The once province of Britain." J. N. Pomeroy.. At once. (a) At the same point of time; immediately; without delay. "Stand not upon the order of your going, but go at once." Shak. "I . . . withdrew at once and altogether." Jeffrey. (b) At one and the same time; simultaneously; in one body; as, they all moved at once. -- Once and again, once and once more; repeatedly. "A dove sent forth once and again, to spy." Milton.