formal

ˈfɔːm(ə)l

adjective

done in accordance with convention or etiquette; suitable for or constituting an official or important occasion.

a formal dinner party

ceremonial,ceremonious,ritualistic,ritual,conventional,traditional,orthodox,prescribed,fixed,set,stately,courtly,solemn,dignified,elaborate,ornate,dressy

noun

an evening dress.

cocktail parties every night so the ladies can show off their formals