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:
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.