A ball is a formal social gathering, where people dance. Like a very posh party. Associated words are: ballroom, ballgown etc An embassy is is usually the place where an ambassador from another country works. In London, it is here: http://www.conspalon.org/ So, the embassy ball is a formal dance party at a foreign ambassador's offices and if you are to attend, you must dress formally (evening dress or ballgown for women, Dinner jacket (tuxedo) for men).