The Collaborative International Dictionary of English v.0.48:

Once \Once\ (w[u^]ns), adv. [OE. ones, anes, an adverbial form
   fr. one, on, an, one. See One-, -Wards.]
   1. For one time; by limitation to the number one; not twice
      nor any number of times more than one.
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            Ye shall . . . go round about the city once. --Josh.
                                                  vi. 3.
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            Trees that bear mast are fruitful but once in two
            years.                                --Bacon.
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   2. At some one period of time; -- used indefinitely.
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            My soul had once some foolish fondness for thee.
                                                  --Addison.
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            That court which we shall once govern. --Bp. Hall.
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   3. At any one time; -- often nearly equivalent to ever, if
      ever, or whenever; as, once kindled, it may not be
      quenched.
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            Wilt thou not be made clean? When shall it once be?
                                                  --Jer. xiii.
                                                  27.
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            To be once in doubt
            Is once to be resolved.               --Shak.
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   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.
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   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.
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