Extreme metal is a loosely defined umbrella term for a number of related heavy metal music subgenres that have developed since the early 1980s. It has been defined as a "cluster of metal subgenres characterized by sonic, verbal, and visual transgression".
The term usually refers to a more abrasive, harsher, underground, non-commercialized style associated with the speed metal, thrash metal, black metal, death metal, and doom metal genres. Hardcore punk has been considered an integral part of the development of extreme metal in song structure and speed, apart from the case of doom metal.
Extreme metal acts set themselves apart from traditional heavy metal acts, such as Iron Maiden, Judas Priest and Motörhead, by incorporating more abrasive musical characteristics such as higher tempos, increased aggression and a harsher extremity. In the majority of the world, extreme metal does not receive much radio-play or achieve high chart positions.
Extreme metal's sonic excess is characterized by high levels of distortion (also in the vocals – growling, gargling or screaming), less focus on guitar solos and melody, emphasis on technical control, and fast tempos (at times, more than 200 beats per minute). Its thematic transgression can be found in more overt and/or serious references to Satanism and the darker aspects of human existence that are considered out of bounds or distasteful, such as death, suicide and war." "Visual transgression [can include] ... medieval weaponry [and] bloody/horrific artwork."
According to ethnographer Keith Kahn-Harris, the defining characteristics of extreme metal can all be regarded as clearly transgressive: the "extreme" traits noted above are all intended to violate or transgress given cultural, artistic, social or aesthetic boundaries. Kahn-Harris states that extreme metal can be "close to being ... formless noise", at least to the uninitiated listener. He states that with extreme metal lyrics, they often "offer no possibility of hope or redemption" and lyrics often reference apocalyptic themes.