at

from The American Heritage® Dictionary of the English Language, 5th Edition.

• preposition In or near the area occupied by; in or near the location of.
• preposition In or near the position of.
• preposition To or toward the direction or location of, especially for a specific purpose.
• preposition Present during; attending.
• preposition Within the interval or span of.
• preposition In the state or condition of.
• preposition In the activity or field of.
• preposition To or using the rate, extent, or amount of; to the point of.
• preposition On, near, or by the time or age of.
• preposition On account of; because of.
• preposition By way of; through.
• preposition In accord with; following.
• preposition Dependent upon.
• preposition Occupied with.
• idiom (at it) Engaged in verbal or physical conflict; arguing or fighting.

from The Century Dictionary.

• An obsolete and dialectal form of that.
• An assimilated form of ad- before t, as in attract, attend, etc.
• A prefix of Anglo-Saxon origin, meaning at, close to, to: common in Middle English, but now obsolete.
• A preposition of extremely various use, primarily meaning to, without implication, in itself, of motion.
• 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.
• 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.
• From (implying a source from which a thing comes or where it is sought): as, to receive ill treatment at their hands.
• 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.
• 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.
• 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.
• [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.]
• With the infinitive: To.
• [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.]

from the GNU version of the Collaborative International Dictionary of English.

• preposition 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.
• preposition A relation of proximity to, or of presence in or on, something
• preposition The relation of some state or condition
• preposition The relation of some employment or action; occupied with
• preposition The relation of a point or position in a series, or of degree, rate, or value
• preposition The relations of time, age, or order
• preposition The relations of source, occasion, reason, consequence, or effect
• preposition Relation of direction toward an object or end
• preposition See under All, Home, Large, Last (phrase and syn.), Length, Once, etc.
• preposition busily or actively engaged.
• preposition See Least and However.
• preposition See At one, in the Vocabulary.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• preposition In or very near a particular place.
• preposition indicating time Simultaneous, during.
• preposition In the direction of (often in an unfocused or uncaring manner).
• preposition Occupied in (activity).