It seems to be more and more confusing to decide what to wear to parties these days. There are multiple terms meaning the same thing being used on invitations. This duplication leads to many gray areas when it comes to what you really should wear to your event.
Here is a quick reference guide to dress code implications for social events listed in the order of informal to formal. I follow this with a quick glance at what constitutes a tuxedo and separates a ball gown from an evening gown.
Casual - This means use your own good judgement. Keep in mind; it never hurts to look a step better than everyone else! Please avoid cliches like Santa ties and reindeer antlers!
Dressy Casual goes along the same lines as business casual but maybe a touch more formal. A nice button down or collared shirt for a man will step it a pair of nice jeans. SO will a sport coat. Some folks say jeans are not appropriate but I think they are fine if they are dark washed, dress jeans. Shorts are definitely not appropriate.
Business Casual - Men should wear a sport coat and slacks, no suit is required. Women should also wear something other than a suit. Dresses or dress slacks with a sweater work well. The overall theme is that you should be neatly dressed and professional in appearance.
Cocktail Attire - Remember, the word cocktail implies fun so try not to dress in your work uniform. Something less than formal but more than business is required. For women this means short, elegant dresses for her and dark suits for him. Elegance and sophistication will give you a step up when you wear a dress. Keep away from flash and glitz, instead go simple and streamlined. Typically, cocktail-wear is knee length, though if you are wearing a simple straight skirt, you can wear a longer hemline. If slacks are your choice, combine them with a sleek and sexy top. Silk blouses or cashmere sweaters will dress up your slacks. With pants or a skirt, be careful to avoid looking like you are going to a business meeting.
Semi-formal - This is the sketchiest of all dress codes. Women can wear a dress, pant suit or dress suit. Great fabrics for women's semi-formal attire are the luxurious fabrics of silk, velvet and cashmere. Usually, anything silky goes. Dresses can be cocktail dresses, no more than an inch above the knee, or they can extend to the ankle. For men, this usually means that tuxes are not required. A wedding scheduled for after 6 PM still requires dark suits for the gent, and a cocktail or short elegant dress for her. Daytime semi-formal events mean a suit for him and an appropriate short dress or dressy suit for her.
Creative Black Tie - You have the latitude to create your own look. A trendy interpretation will combine a tuxedo with a black or colored shirt and no tie. Fashion trends can sometimes dictate that various color selections for the pants and jacket of the tuxedo are appropriate, She can wear a short or long dress or maybe a long skirt with sequins or lace trim and combine it with a lightweight cashmere sweater.
Black Tie Invited - This means that you don't necessarily need to wear a tuxedo, but it does let you know that the event is indeed a formal affair. A dark suit and tie is another option. Women should wear long dresses, cocktail gowns cut to the shin or combine separates that are festive and glitzy.
Semi Formal Dress - For women, semi formal mean anything from a dressy business suit to a cocktail dress and never a ball gown. The golden rule of the semi formal dress: keep the hemline to the knees or lower, but never touch the ground. Men will wear dark suit and tie or dress slacks and nice shirt with blazer/ jacket.
Black Tie Optional -This indicates that men should wear a dark suit and tie or use the option of wearing their tuxedo. The darker the suit color, the better. Black, dark gray, super deep navy. Your shirt/tie should not be playful or made with crazy patterns, to follow the theme of black tie seriousness. I found a great quote online about this, and although appears tongue in cheek it does include a waft of reality: "Black tie optional" is passive aggressive. It means, "If you don't think we are worth going to the trouble of wearing a tux, we won't kick you out, but are we really asking too much that you dress up nicely for once?!"