Cocktail. A mix between traditional semi-formal and black tie. Avoid casual dress such as khakis, sweater dresses/sun dresses, etc. but formal gowns and tuxedos are not required.
For women: short cocktail dresses, formal jumpsuits, and floor-length gowns are all great options.
For men: anything from dress pants, button up, and ties to formal tuxes are perfectly fine.