at

American Heritage® Dictionary of the English Language, Fourth Edition

1. prep. In or near the area occupied by; in or near the location of: at the market; at our destination.
2. prep. In or near the position of: always at my side; at the center of the page.
3. prep. To or toward the direction or location of, especially for a specific purpose: Questions came at us from all sides.
4. prep. Present during; attending: at the dance.
5. prep. Within the interval or span of: at the dinner hour; at a glance.
6. prep. In the state or condition of: at peace with one's conscience.
7. prep. In the activity or field of: skilled at playing chess; good at math.
8. prep. To or using the rate, extent, or amount of; to the point of: at 30 cents a pound; at high speed; at 20 paces; at 350°F.
9. prep. On, near, or by the time or age of: at three o'clock; at 72 years of age.
10. prep. On account of; because of: rejoice at a victory.
11. prep. By way of; through: exited at the rear gate.
12. prep. In accord with; following: at my request.
13. prep. Dependent upon: at the mercy of the court.
14. prep. Occupied with: at work.
15. idiom. at it Informal Engaged in verbal or physical conflict; arguing or fighting: The neighbors are at it again.
16. n. See Table at currency.

Century Dictionary and Cyclopedia

1. A preposition of extremely various use, primarily meaning to, without implication, in itself, of motion. It expresses position attained by motion to, and hence contact, contiguity, or coincidence, actual or approximate, in space or time. Being less restricted as to relative position than other prepositions, it may in different constructions assume their office, and so become equivalent, according to the context, to in, on, near, by, about, under, over, through, from, to, toward, etc.
2. Of simple local position: With verbs of rest (be, live, etc.): In, on, near, by, etc., according to the context: denoting usually a place conceived of as a mere point: as, at the center, at the top, at the corner, at the end, at the next station, at the bend of the river, at the north pole, at No. 48 Main street, etc. So with names of towns, etc.: as, at Stratford, at Lexington, etc.; but if the city is of great size in is commonly used: as, in London, in Paris, in New York; unless, again, the city is conceived of as a mere geographical point: as, our financial interests center at New York. The place implied by at may be left indeterminate, with a reference rather to condition than to mere location: as, at school, at college, at court, at sea, etc. At may also express personal proximity: as, at one's side, at one's heels, at one's elbow, etc. At hand, near by, has lost its personal reference.
3. With verbs of motion: Through, by (implying a starting-point or a point where a thing enters or departs): as, to enter at the window, to go out at the back door.
4. From (implying a source from which a thing comes or where it is sought): as, to receive ill treatment at their hands.
5. To, toward (implying a stopping-point, a position attained or aimed at): as, to come at, to get at, to aim at, fire at, shoot at, drive at, point at, look at, shout at, reach at, snatch at, clutch at, etc.; also be at when it implies effort directed toward a thing.
6. Of circumstantial position, state, condition, manner, environment, etc., in a great variety of relations developed from the local sense: as, at dinner, at play, at work, at service, at right angles, at full length, at odds, at ease, at war, at peace, at will, at pleasure, at discretion, etc.
7. Of relative position: implying a point in an actual or possible series, and hence used of degree, price, time, order, occasion, etc.: as, at the beginning, at the third house from the corner, at nine years of age, at seventy degrees in the shade, at four dollars a yard, at ten cents a pound, at half past six, at midnight, at first, at last, etc.
8. [In all uses, especially in those last mentioned, at is very frequent in idiomatical phrases: as, at all, at most, at least, at last, at length, at any rate, at stake, at one, at once, at large, at present, etc., for which see the principal words, all, most, least, etc.]
9. With the infinitive: To.
10. [Now only dialectal, but common in Middle English, and the regular use in Scandinavian, to which the English use is due. A relic of this use remains in ado, originally at do. See ado.]
11. An obsolete and dialectal form of that.
12. A prefix of Anglo-Saxon origin, meaning at, close to, to: common in Middle English, but now obsolete. A relic of it remains in twit, originally at-wite. In atone, the at- is not properly a prefix, but is the preposition merged with its object.
13. An assimilated form of ad- before t, as in attract, attend, etc.

Wiktionary

1. prep. In or very near a particular place.
2. prep. indicating time Simultaneous, during.
3. prep. In the direction of (often in an unfocused or uncaring manner).
4. prep. Occupied in (activity).
5. prep. Indicates a position on a scale or in a series.
6. prep. Because of.
7. prep. Holding a given speed or rate.
8. prep. In a state of.
9. n. the @ symbol.

GNU Webster's 1913

1. prep. Primarily, this word expresses the relations of presence, nearness in place or time, or direction toward; 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.
2. prep. A relation of proximity to, or of presence in or on, something
3. prep. The relation of some state or condition
4. prep. The relation of some employment or action; occupied with
5. prep. The relation of a point or position in a series, or of degree, rate, or value
6. prep. The relations of time, age, or order
7. prep. The relations of source, occasion, reason, consequence, or effect
8. prep. Relation of direction toward an object or end

WordNet 3.0

1. n. a highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium
2. n. 100 at equal 1 kip in Laos