1. To be located somewhere: The hotel is at the center of town. I am at the corner of 11th Street and Third Avenue.
2. To take place at some given time: Our meeting is at noon. The movie is at 8:00, so we've got plenty of time for dinner.
3. To be busy trying to accomplish something; keep at. Often used with it: I have been at this interminable project for weeks now.
4. To remind someone regularly about something or to do something: He's been at me about the book he lent to me.