Hard rock music, usually electric guitar-based and always loud.
'Heavy metal' seems at first a strange label to apply to a form of music. However, a little investigation into the symbolism behind the name reveals it to be a rather obvious choice.
'Heavy' was coined in the beatnik era of the 1950s to mean serious or profound and the term 'heavy music' was then and later applied to music in that vein. It's clear to see this meaning of heavy is derived from the usual meaning, that is, weighty or massive.
Okay, that's 'heavy' explained but why should a form of music be called metal? Well, most metals are heavy, especially the metals favoured by the bands who played that genre and used metals in their names, for example Led Zeppelin, Iron Butterfly, Iron Maiden and Metallica. Also, the term 'heavy metals' in the chemical sense includes mercury, lead and cadmium, which have just the right image of toxicity to suit the musical style. So, both 'heavy' and 'metal' are suitable candidate words for this genre. Add to that the fact that heavy metal had already been widely used as a military term for heavily fortified tanks/guns etc. and it starts to look like an ideal choice as a label.
The first heavy metal bands, notably Black Sabbath/Ozzy Osbourne, Led Zeppelin and Judas Priest, hailed from Birmingham, which was then (sadly no longer) a principal centre of metal goods manufacturing in the UK. With the decline of that manufacturing tradition, most of the 'metal bashing', as it was known, is now done by these bands rather than by men with big hammers.
Birmingham was the first industrialised city in the world and this industry was centred around the factories and furnaces of the aptly named Black Country, described by the US Consul to the city in 1868 as "Black by day, red by night". Many of the members of the above groups worked in factories by day (Ozzy Osbourne was, for example, a horn tester in a car factory) and played in hard rock bands by night. It's hardly surprising that the music reflected the sights and sounds that the band members were surrounded by. Black Sabbath are generally accepted to be the first heavy metal band. They were originally called Earth after J. R. R Tolkien's mythical Middle Earth. Tolkien was a local boy and is thought to have based the region of Mordor on the Black Country. All in all, it's clear where the musical form heavy metal originated.
Although Ozzy and friends were playing heavy metal in the late 1960's/early 1970s, they didn't use that expression to describe it - for that we need to cross to the USA. The term 'heavy metal' first appears in print in William Burroughs' 1962 novel The Soft Machine. His character Uranian Willy is described as "the Heavy Metal Kid". Burroughs later re-used the term in his 1964 novel Nova Express:
"With their diseases and orgasm drugs and their sexless parasite life forms - Heavy Metal People of Uranus wrapped in cool blue mist of vaporized bank notes - And the Insect People of Minraud with metal music."
The term 'heavy metal' was used in a musical context in the title of a 1967 album by the British avant-garde outfit Hapshash and the Coloured Coat - Featuring The Human Host And The Heavy Metal Kids. The title wasn't applied to a particular musical style and appears to have been a reference to the 'kids' in Burroughs' novel.
It isn't clear who first appropriated the term to refer to loud rock music, although several lay claim to it. The widely quoted description of Jimi Hendrix's music as 'like listening to heavy metal falling from the sky', while being a fairly accurate assessment, isn't the earliest use of the phrase.
Some claim that the US rock music critic Lester Bangs, while working for Creem magazine, used the expression in 1968 to describe a performance of the band MC5 (Motor City Five), but I've been unable to find documentary evidence of that. Creem magazine themselves attribute the term to Mike Saunders, in an article about the 'Kingdom Come' album, by Sir Lord Baltimore, in the May 1971 edition of the magazine:
"This album is a far cry from the currently prevalent Grand Funk sludge, because Sir Lord Baltimore seems to have down pat most all the best heavy metal tricks in the book."
This has the benefit of being a traceable citation, as copies of the edition are still extant. So, until other hard evidence is found, that has to be the current strongest claim. It would be surprising if the term had never been used in the musical context before 1971 though - after all Steppenwolf used it in the lyric of their 1968 song Born to be Wild:
"I like smoke and lightning
Heavy metal thunder
Racin' with the wind
And the feelin' that I'm under"
The phrase may be American but the music was born, like my good self, in the 'jewel of the English Midlands' - the Black Country.
See other phrases that were coined in the USA.