Things have changed through the years. You will have people who dressed to the 9s for "formal nights", but really, any type of evening wear, buisness attires, cocktail dresses, gowns will be suitable for ladies and a suit and tie for men is sufficient. Formals does not mean long gown for ladies. It hasn't been for years.

The rest of the time, "smart causual" is the operative term and it would be items with collars and sleeves for men (golf shirt etc), no tees, no tanks; and similar for ladies.

Here is a link to some FQA for first timers. Hope it's helpful.
http://www.cruisecritic.com/features/area.cfm?ID=13

Seeing as you tagged your question under "Cunard", I'd advise you to go over to the Cunard board/forum on Cruise Critic and ask your question there. Over the years the dress code has relaxed somewhat on most cruise lines, but I believe Cunard is a fairly unique "kettle of fish" when it comes to dress code.
I've read posts over there about hats, ascots, shirts with logos, upmarket clothing - perhaps you might be safer to discuss all your options over there, with people who are regular Cunard cruisers.

Thanks for the replies. I agree that Cunard seems to be unique in its dress code and thus my query. I'm thinking that a short dress would likely be fine for formal nights if it is sufficently dressy but will check on Cruise Critic. However, I refuse to do a hat I believe that might be one of their special events which can be passsed up if one wants. Will check anyway.

Thanks

Cunard is, IMO, no more "unique" in its dress code than some of the other cruise lines. If you read THEIR information carefully you'll see they all would LOVE to have people show up in long gowns and tuxes but that just isn't happening anymore the way it used to (at least when I first cruised back in the early 1980's).

The Cunard information I read was quite clear that a tux OR a dark suit, etec., is perfectly acceptable

And let's face facts here...if you've goot decent legs and a great smile, NOBODY, except perhaps some jealous other woman, is going to

keep you out of the party
throw you over the side
do anything to ruin your cruise

GO for it