at

In, near, or in the general vicinity of (a particular place).

• Caesar was at Rome.
• A climate treaty was signed at Kyoto in 1997.
• I was at Jim's house at the corner of Fourth Street and Vine.

Attending (an educational institution).

• She's at Oxford University, studying chemistry.

Working for (a company) or in (a place or situation).

• He used to be at Lehman Brothers. Now he's at Merrill Lynch.
• 'Where does he work?' — 'I think he's still at the solicitors.'

Indicating distance or direction relative to the speaker.

• Target at five miles. Prepare torpedoes.
• Look out! UFO at two o'clock!

Present or taking place during (an event).

• Was he at the meeting?
• There was a big fight at the class reunion.

Indicating time of occurrence, especially an instant of time, or a period of time relatively short in context or from the speaker’s perspective.

• at six o’clock; at dawn; at closing time; at the age of twelve; at night; at the moment

On (a particular date).

In the direction of; towards; (often implied to be in a hostile or careless manner).

• Don’t just talk at someone; really listen to what they have to say.
• Stones were thrown at us, but we couldn't see who did.
• He shouted at her.

Indicating action bearing upon something, especially continued or repeated action.

• Don't pick at your food!
• My cat keeps scratching at the furniture.
• I was working at the problem all day.

In response or reaction to.

• At my request, they agreed to move us to another hotel.
• He jumped at the sudden noise.
• We laughed at the joke.

Occupied in (activity).

• men at work; children at play

In a state of.

• The two countries are at war.
• She is at sixes and sevens with him.
• They are at loggerheads over how best to tackle the fiscal cliff.

Subject to.

• We hope that the event will go ahead, but we are at the whim of the weather.
• The city was at the mercy of the occupying forces.

Denotes a price.

• 3 apples at 2¢ (each)
• The offer was at $30,000 before negotiations.

Indicates a position on a scale or in a series.

• Sell at $90.
• Tiger finished the round at tenth, seven strokes behind the leaders.
• I’m offering it—just to select customers—at cost.

In certain phrases, used to indicate the manner in which something happens or is done.

• The car came towards me at speed.
• He spoke at great length on the topic.
• The winner will be chosen at random.

Indicates a specific speed or rate that is maintained by something.

• We were cruising along at well over the speed limit of fifty miles per hour.
• It is growing at the rate of 3% a year.

Indicates a means or method.

• 'How was the painting sold?' — 'At auction.'

The at sign (@).

Rare form of @; to reply to or talk to someone, either online or face-to-face.

Alternative form of 'at (relative pronoun; reduced form of "that" and/or "what").

Alternative form of att (Laos currency unit)