From The Collaborative International Dictionary of English v.0.48 [gcide]:
At \At\, preposition [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:
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.
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.
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.
From WordNet (r) 3.0 (2006) [wn]:
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