What's the difference between cocktail and formal attire?

Cocktail attire features knee-length or midi dresses and dressy separates, while formal attire requires floor-length gowns or tuxedos. The key difference is length—cocktail hems stop at or above the knee, whereas formal attire calls for full-length silhouettes. Formal attire also represents a higher level of dressiness overall, making it appropriate for black-tie events where cocktail attire would be underdressed.