Heavy metal (or simply metal) is a genre of rock music that developed in the late 1960s and early 1970s, largely in the United Kingdom and United States. With roots in blues rock, psychedelic rock and acid rock, heavy metal bands developed a thick, monumental sound characterized by distorted guitars, extended guitar solos, emphatic beats and loudness.
During the mid-1970s, Judas Priest helped spur the genre's evolution by discarding much of its blues influence, while Motörhead introduced a punk rock sensibility and an increasing emphasis on speed. Beginning in the late 1970s, bands in the new wave of British heavy metal such as Iron Maiden and Saxon followed in a similar vein.
During the 1980s, glam metal became popular with groups such as Mötley Crüe and Def Leppard, however, underground scenes produced an array of more aggressive styles: thrash metal broke into the mainstream with bands such as Metallica, Slayer, Megadeth and Anthrax, while other extreme subgenres such as death metal and black metal became – and remain – subcultural phenomena. Since the mid-1990s, popular styles have expanded the definition of the genre. These include groove metal and nu metal, the latter of which often incorporates elements of grunge and hip-hop.