For men, the definition of cocktail attire is relatively simple, although there is room for variation. A dark suit or well-pressed pants, sport coat, and a crisp dress shirt and tie is sufficient for most occasions. For less formal events, a man might not wear a jacket or, alternately, wear a suit without a tie.
Cocktail attire for men used to mean a suit with a jacket and tie; lately however, men have been ditching the tie and going for a fitted suit, with a button down shirt (top button undone). If you stick to a suit jacket and tie, one suggestion is to go with a skinny tie and a straight-leg fitted pant (not skinny pants). Also, remember that cocktail suits are your time to be flashy - you are allowed to be creative in your suit!
Tuxedos are not really considered cocktail attire. A blazer, a collared shirt, and khakis are other choices that can be considered.
There is more range in cocktail attire for women, although there are a few standard guidelines. Hemlines for dresses and skirts are usually at or slightly above the knee. Long, dressy pants with a silk blouse or cashmere sweater would also be appropriate.
The traditional choice is a cocktail dress. Since such dresses can range from strapless and flashy to a more conservative little black dress, a woman should be able to find a look she's comfortable with. It's usually best to choose an outfit that is more elegant than showy, and isn't too short or too revealing. Again, the context of the event is important, and it's usually better to be a little conservative than to be too flashy.
Typically, cocktail wear is knee length, though if a woman is wearing a simple straight skirt, longer is acceptable. A skirt and jacket or evening suit could be worn, especially if paired with dressy top. Cocktail attire usually suggests fun, so a woman shouldn't look like she is going to a business meeting.