When a woman is invited to a cocktail party, she just has to throw on a cocktail dress to know she'll fit right in. Men's clothing isn't so conveniently named, though. Thinking of a cocktail party as requiring dressy-casual attire gives you an idea of what to wear.
Dressy Casual Defined
You'll never go wrong with slacks and a blazer or sport coat, but be mindful of the time of year. Wear wool or tweed in cold-weather months, and opt for linen, summer-weight wool or madras in warmer months. Choose a shirt with a collar, rather than a T-shirt. Casual button-downs, a dress shirt worn with an open collar or even a polo shirt will do. Ties are completely optional.
Consider the venue and time of day when you choose your clothing too. A swanky club or restaurant reserved for a private party means you can break away from staid grays and browns and add patterns or color. If the event is more serious in nature, such as an office soiree, stick to classic colors and reserved patterns. Daytime cocktail events give you the opportunity to be a bit more relaxed. You might pull out the plaid or seersucker slacks or jacket for a garden party.