If the invitation states, Black Tie Preferred, Black Tie Optional or Black Tie Invited, a traditional tuxedo is the preferred choice of attire for the gentleman.  If he decides not to wear a tuxedo, he may wear a dark suit, white dress shirt and conservative tie, which is the next step down from Black Tie Preferred.  Anything less formal than this would be unacceptable at this event!  

The lady accompanying a gentleman who is wearing a suit instead of a tuxedo may wear either a short evening gown or a dinner suit.  A dinner suit is a suit that is made of a finer material than a business-suit and may have embellishments on it such as fancy buttons.