Anonymous wrote:technically, ball = ball gown = full length, full skirted dress
Since it is pretty tough to find a true ball gown at a reasonable price, any floor length gown will work. Of course, a ball is also white tie, not black tie.
More realistically, I'd see if I could find pictures from the past two inaugural balls thrown by the same group. That will give you a good idea of what is standard for that group.
This is DC. If the ball is not "Official" wear a cocktail dress if you want. No one will care.