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 or sound associated with the doom metal, speed metal, thrash metal, black metal, and death metal genres.