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.

At \At\, prep. [AS. [ae]t; akin to OHG. az, Goth., OS., & Icel.
   at, Sw. [*a]t, Dan. & L. ad.]
   Primarily, this word expresses the relations of presence,
   nearness in place or time, or direction toward; as, at the
   ninth hour; at the house; to aim at a mark. It is less
   definite than in or on; at the house may be in or near the
   house. From this original import are derived all the various
   uses of at. It expresses: 

   1. A relation of proximity to, or of presence in or on,
      something; as, at the door; at your shop; at home; at
      school; at hand; at sea and on land.

   2. The relation of some state or condition; as, at war; at
      peace; at ease; at your service; at fault; at liberty; at
      risk; at disadvantage.

   3. The relation of some employment or action; occupied with;
      as, at engraving; at husbandry; at play; at work; at meat
      (eating); except at puns.

   4. The relation of a point or position in a series, or of
      degree, rate, or value; as, with the thermometer at
      80[deg]; goods sold at a cheap price; a country estimated
      at 10,000 square miles; life is short at the longest.

   5. The relations of time, age, or order; as, at ten o'clock;
      at twenty-one; at once; at first.

   6. The relations of source, occasion, reason, consequence, or
      effect; as, at the sight; at this news; merry at anything;
      at this declaration; at his command; to demand, require,
      receive, deserve, endure at your hands.

   7. Relation of direction toward an object or end; as, look at
      it; to point at one; to aim at a mark; to throw, strike,
      shoot, wink, mock, laugh at any one.

   {At all}, {At home}, {At large}, {At last}, {At length}, {At
   once}, etc. See under {All}, {Home}, {Large}, {Last} (phrase
      and syn.), {Length}, {Once}, etc.

   {At it}, busily or actively engaged.

   {At least}. See {Least} and {However}.

   {At one}. See {At one}, in the Vocabulary.

   Syn: {In}, {At}.

   Usage: When reference to the interior of any place is made
          prominent in is used. It is used before the names of
          countries and cities (esp. large cities); as, we live
          in America, in New York, in the South. At is commonly
          employed before names of houses, institutions,
          villages, and small places; as, Milton was educated at
          Christ's College; money taken in at the Customhouse; I
          saw him at the jeweler's; we live at Beachville. At
          may be used before the name of a city when it is
          regarded as a mere point of locality. ``An English
          king was crowned at Paris.'' --Macaulay. ``Jean
          Jacques Rousseau was born at Geneva, June, 28, 1712.''
          --J. Morley. In regard to time, we say at the hour, on
          the day, in the year; as, at 9 o'clock, on the morning
          of July 5th, in the year 1775.

Source : WordNet®

at once
     adv 1: without delay or hesitation; with no time intervening; "he
            answered immediately"; "found an answer straightaway";
            "an official accused of dishonesty should be suspended
            forthwith"; "Come here now!" [syn: {immediately}, {instantly},
             {straightaway}, {straight off}, {directly}, {now}, {right
            away}, {forthwith}, {in real time}, {like a shot}]
     2: simultaneously; "he took three cookies at a time" [syn: {at
        a time}, {at one time}]