People like to dress appropriately for an event, be it a dinner out with friends or an important wedding. But host and hostesses should be clear when issuing invitations which the desired dress is. I've received many ambiguous related invitations over the years and wanted to help others dicipher the vague dress-requests that they may receive.
-If you're not sure how to dress, over dress for an event. Many people interrupt wearing for an event. You'll feel less obvious if you over -ress.
-Formal. Black tie (tuxedo) for men, floor length gowns for women.
-Black tie preferred, optional or invited. Tuxedo is the preferred choice for men, if the man does wear a tux then the women should be in a formal floor length gown. However if the man does not wear a tux, he will wear a dark suit, white shirt and conservative tie. A women in this case will wear a short dinner gown or an evening suit, a skirt with matching jacket.
-Dressy casual. Dress pants with matching jackets but no tie for men and dresses or dark pantsuits for women.
-Business casual. Pressed dress pants or khaki twills with a collared shirt, but no tie. Never wear jeans to a business casual event. Socks and belts required for men. Women can wear pants or skirts with hose and a collared shirt. No flip-flops.
-Casual. Usually anything goes, but if you have never been with the host or hostess and it says casual, it's better to over-dress until you have a better understanding of how they define casual.