Real Dictionary

avant-garde
Avant-garde art, music, theatre, and literature is very modern and experimental.

avant-garde in American English
of such movements, ideas, etc.
the leaders in new or unconventional movements, esp. in the arts; vanguard
adjective: of or pertaining to the experimental treatment of artistic, musical, or literary material
noun: the advance group in any field, esp. in the visual, literary, or musical arts, whose works are characterized chiefly by unorthodox and experimental methods

avant-garde in British English
adjective: of such artists, etc, their ideas, or their techniques
noun: those artists, writers, musicians, etc, whose techniques and ideas are markedly experimental or in advance of those generally accepted