A masquerade ball (or bal masqué) is an event which the participants attend in costume wearing a mask. (Compare the word "masque" - a formal written and sung court pageant.)
Such gatherings, festivities of Carnival, were paralleled from the 15th century by increasingly elaborate allegorical Entries, pageants and triumphal processions celebrating marriages and other dynastic events of late medieval court life. Such costumed dances were a special luxury of the ducal court of Burgundy.
Masquerade balls were extended into costumed public festivities in Italy during the 16th century Renaissance (Italian, maschera). They were generally elaborate dances held for members of the upper classes, and were particularly popular in Venice. They have been associated with the tradition of the Venetian Carnival. With the fall of the Venetian Republic at the end of the 18th century, the use and tradition of masks gradually began to decline, until they disappeared altogether.
A new resurgence of masquerade balls began in the late 1990s in North America and are still held today, though in modern times the party atmosphere is emphasized and the formal dancing usually less prominent. Less formal "costume parties" may be a descendant of this tradition.