Source : Webster's Revised Unabridged Dictionary (1913)

Once \Once\, adv. [OE. ones, anes, an adverbial form fr. one,
   on, an, one. See {One-}, {-Wards}.]
   1. By limitation to the number one; for one time; not twice
      nor any number of times more than one.

            Ye shall . . . go round about the city once. --Josh.
                                                  vi. 3.

            Trees that bear mast are fruitful but once in two
            years.                                --Bacon.

   2. 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. 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.
                                                  27.

            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.

Source : WordNet®

once
     adj : belonging to some prior time; "erstwhile friend"; "our
           former glory"; "the once capital of the state"; "her
           quondam lover" [syn: {erstwhile(a)}, {former(a)}, {once(a)},
            {onetime(a)}, {quondam(a)}, {sometime(a)}]

once
     adv 1: on one occasion; "once I ran into her" [syn: {one time}, {in
            one case}]
     2: as soon as; "once we are home, we can rest" [syn: {when}]
     3: at a previous time; "once he loved her"; "her erstwhile
        writing" [syn: {formerly}, {at one time}, {erstwhile}, {erst}]