Wedding attire differs from State to State but your description was perfect. Long or short dress or Formal pants outfit, silk, velvet, sparkly or sheer, all fit the description of nice formal evening attire. Classic black evening palazzo pants are very popular as is the basic black cocktail dress. Dress your oufit up with jewelry and stapy sandals and you will be just fine.
I've looked through all the posts and everyone assumes we know what is ok for Ladies Formal - I however am not sure.
Standards do vary somewhat from line to line, so the best advice always is to follow the guidelines in the pre-cruise information package that your travel agent will deliver to you thirty to sixty days before the cruise (yes, this means that you need to take the time to sit down and read naterials in the information package carefully before you pack...). If you follow the cruise line's guidance, you won't be embarrassed.
Would there be a problem wearing a dress or pantsuit that you would wear as a guest to a wedding or to church on the formal nights?
In my circles, most guests at weddings seem to wear "informal" or "semiformal" attire whereas a true "formal" evening demands "black tie" or equivalent -- which would mean either a formal evening gown (Donna's "prom dress") or a formal cocktail dress for ladies. Although many cruise lines opted for a modified formal standard that admits dark business suits in addition to tuxedos or dinner jackets for gentlemen (originally as a concession to the fact that many gentlemen did not own tuxedos, but did own business suits, back in the 1970's), the attire for ladies generally has remained very dressy -- espeically on the "premium" lines like Princess Cruises, Holland America Lines, and Celebrity Cruises.
I think that most of the major cruise lines list formal pant suits (the silk or satin kind that look like fancy pajamas) as an acceptable option for women, but they seem to be waning in popularity. On my last several cruises (all with Princess), I have seen very few women wearing htem.