Avant-garde metal or experimental metal is a subgenre of heavy metal music characterised by the use of innovative, avant-garde elements, large-scale experimentation, and the use of non-standard sounds, instruments, and song structures. The earliest avant-garde metal bands include Celtic Frost and Master's Hammer.
Avant-Garde Metal cannot be treated like other genres, such as Black, Death, Doom, Thrash, etc. Those genres are rooted in similar aesthetics and ideologies, leading to strong commonalities amongst bands united under one genre. With the Avant-Garde, on the other hand, there really is no common aesthetic or ideology. The bands have to be taken on a one-by-one basis, evaluated individually rather than on genre aesthetics.
The term avant-garde metal refers to bands and musicians who "incorporate new and innovative elements in metal, who break conventions, tear down walls, violate borders." The genre has also been described as "the art of creating deep and strange atmospheres by experimenting with new instruments and sounds, strange vocals, unconventional song structures, rhythms and harmonies, unusual lyrics or uncommon artwork" or alternatively, "progressive, psychedelic, surrealistic, phantasmagoric, expressionistic, dissonant or extravagant interpretations of extreme metal."
Michael Haas of Angizia notes that avantgarde is "a conscious distance from traditional listening and composing habits" while Svein Egil Hatlevik of Fleurety identifies avant-garde metal as "an aesthetic ideology" to "make music that's more than just average metal." He also notes that heavy metal music is a "field where it still makes sense to be avantgarde" because it is "one of the most conservative fields of artistic practice in the world." Not everyone agrees with the use of the term to establish a subgenre of metal, however. Jeff Arwadi of Kekal warns that "when another sub-genre has become established, it would create nothing but limits, and I don't think it's wise if we try to establish sub-genre [sic] that limits creativity and expression." Chlordane of The Amenta takes issue with the tendency in heavy metal music for "anyone who does something slightly weird" to be considered avant-garde. He contends that the term should not be used unless a band is "pushing music forward," further suggesting that the mere use of classical music in heavy metal is not avant-garde as it "has been done," "is not new" and "offers nothing."
Although progressive metal and avant-garde metal both favor experimentation and non-standard ideas, there are rather large differences between the two genres. The experimentation of progressive metal lies mostly in playing complex rhythms and song structures with traditional instruments. For avant-garde metal, most of the experimentation is in the use of unusual sounds and instruments. Progressive metal also puts a greater emphasis on technicality and theoretical complexity (e.g., odd time signatures, complex song forms, jazz fusion influences), while avant-garde metal is more unorthodox and tends to question many of the musical conventions.