>>Rotonda is, AFAIK, the Italian for the original building. The name has become a sort of generic name for any similar building, including the one Jefferson built in Charlottesville - which is also spelled with an U.

From the latin rotundus = round.

Rotunda can refer to a round building, usually with a dome, or to a round room. The church I attended as a child had a large, round room (with no dome) that was called the rotunda.

Rotund is an interesting variation, usually referring to someone's shape, and is a "nice" substitution for "fat," as in:

The rotund gentleman's belly indicated that he had eaten too much jelly.

or

My dear, I just love your pleasant rotundity. (just hope she doesn't have a dictionary nearby with which to hit you).