[ noun ] a party of guests wearing costumes and masks <noun.group>
Masque \Masque\, n. A mask; a masquerade.
She made this 'masque for dancing' in 1931 - Vaughan Williams insisted that his score was not for 'a ballet' - and in it we see, still powerful, the pre-eminent qualities of her choreography.
An important feature of Dido and Aeneas is that Purcell wrote it to be sung all the way through, but that effect is lost if the Prologue can only be performed like a masque, as spoken drama with music.