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. A little investigation into the symbolism behind makes it seem a rather obvious choice though.
'Heavy' was coined in the beatnik area of the 1950s to mean serious or profound. The term 'heavy music' was then and later applied to music that was in that vein. Of course it's clear to see that meaning of heavy is derived from the usual meaning, i.e. weighty or massive.
Okay, that's heavy but why should a form of music be called metal? Well, metal is heavy, especially the metals favoured by the bands who played that genre, e.g. Led Zeppelin, Iron Butterfly and Quicksilver Messenger Service (quicksilver is mercury). Also, the term 'heavy metals' in the chemical sense include mercury, lead and cadmium, which have just the right image of toxicity to suit the musical style. It's interesting, although probably just co-incidence, that many of the British heavy metal bands came from the two principal centres of metal manufacturing in the UK, namely Birmingham (e.g. Led Zeppelin, Black Sabbath/Ozzy Osbourne) and Sheffield (Def Leppard). 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.
So, heavy and metal are ideal candidate words for this genre. Add that to 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 expression 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."
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.
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) from Detroit. 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. Precisely, they sound like a mix between the uptempo noiseblasts of Led Zeppelin (instrumentally) and singing that’s like an unending Johnny Winter shriek: they have it all down cold, including medium or uptempo blasts a la LZ, a perfect carbon of early cataclysmic MC5."
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 musical style remains popular, although less so than in its heyday - the 1980s, and has spawned sub-genres. These include 'death metal', 'thrash metal', 'grindcore' and even 'folk metal' (aka 'heavy wood').
See other phrases that were coined in the USA.