The Collaborative International Dictionary of English v.0.48:

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