According to Answers.com, a "coffee" is an informal social gathering at which coffee and other refreshments are served.
Evidently, this is well known. After all, the New York Times used the term twice in this article without defining it.
- “If you ask my wife, we had the first coffee for Barack,” Rabbi Wolf said."
- "Their paths have crossed sporadically since then, at a coffee Mr. Ayers hosted for Mr. Obama’s first run for office."