The origin of the term "heavy metal" in a musical context is uncertain. The phrase has been used for centuries in chemistry and metallurgy, where the periodic table organizes elements of both light and heavy metals (e.g., uranium). An early use of the term in modern popular culture was by countercultural writer William S. Burroughs. His 1962 novel The Soft Machine includes a character known as "Uranian Willy, the Heavy Metal Kid". Burroughs' next novel, Nova Express (1964), develops the theme, using heavy metal as a metaphor for addictive drugs: "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". Inspired by Burroughs' novels, the term was used in the title of the 1967 album Featuring the Human Host and the Heavy Metal Kids by Hapshash and the Coloured Coat, which has been claimed to be its first use in the context of music. The phrase was later lifted by Sandy Pearlman, who used the term to describe The Byrds for their supposed "aluminium style of context and effect", particularly on their album The Notorious Byrd Brothers (1968).
Metal historian Ian Christe describes what the components of the term mean in "hippiespeak": "heavy" is roughly synonymous with "potent" or "profound," and "metal" designates a certain type of mood, grinding and weighted as with metal. The word "heavy" in this sense was a basic element of beatnik and later countercultural hippie slang, and references to "heavy music"—typically slower, more amplified variations of standard pop fare—were already common by the mid-1960s, such as in reference to Vanilla Fudge. Iron Butterfly's debut album, released in early 1968, was titled Heavy. The first use of "heavy metal" in a song lyric is in reference to a motorcycle in the Steppenwolf song "Born to Be Wild", also released that year: "I like smoke and lightning/Heavy metal thunder/Racin' with the wind/And the feelin' that I'm under."
The first documented use of the phrase to describe a type of rock music identified to date appears in a review by Barry Gifford. In the May 11, 1968, issue of Rolling Stone, he wrote about the album A Long Time Comin' by U.S. band Electric Flag: "Nobody who's been listening to Mike Bloomfield—either talking or playing—in the last few years could have expected this. This is the new soul music, the synthesis of white blues and heavy metal rock." In January 1970 Lucian K. Truscott IV reviewing Led Zeppelin II for the Village Voice described the sound as "heavy" and made comparisons with Blue Cheer and Vanilla Fudge.
Other early documented uses of the phrase are from reviews by critic Mike Saunders. In the November 12, 1970 issue of Rolling Stone, he commented on an album put out the previous year by the British band Humble Pie: "Safe as Yesterday Is, their first American release, proved that Humble Pie could be boring in lots of different ways. Here they were a noisy, unmelodic, heavy metal-leaden shit-rock band with the loud and noisy parts beyond doubt. There were a couple of nice songs ... and one monumental pile of refuse". He described the band's latest, self-titled release as "more of the same 27th-rate heavy metal crap".
In a review of Sir Lord Baltimore's Kingdom Come in the May 1971 Creem, Saunders wrote, "Sir Lord Baltimore seems to have down pat most all the best heavy metal tricks in the book". Creem critic Lester Bangs is credited with popularizing the term via his early 1970s essays on bands such as Led Zeppelin and Black Sabbath. Through the decade, heavy metal was used by certain critics as a virtually automatic putdown. In 1979, lead New York Times popular music critic John Rockwell described what he called "heavy-metal rock" as "brutally aggressive music played mostly for minds clouded by drugs", and, in a different article, as "a crude exaggeration of rock basics that appeals to white teenagers".
Coined by Black Sabbath drummer Bill Ward, "downer rock" was one of the earliest terms used to describe this style of music and was applied to acts such as Sabbath and Bloodrock. Classic Rock magazine described the downer rock culture revolving around the use of Quaaludes and the drinking of wine. Later the term would be replaced by "heavy metal".
Earlier on, as "heavy metal" emerged partially from the heavy psychedelic rock scene, also known as acid rock, "acid rock" was often used interchangeably with "heavy metal" and "hard rock". Musicologist Steve Waksman stated that "the distinction between acid rock, hard rock, and heavy metal can at some point never be more than tenuous", while percussionist John Beck defined "acid rock" as synonymous with hard rock and heavy metal.
Apart from "acid rock", the terms "heavy metal" and "hard rock" have often been used interchangeably, particularly in discussing bands of the 1970s, a period when the terms were largely synonymous. For example, the 1983 Rolling Stone Encyclopedia of Rock & Roll includes this passage: "known for its aggressive blues-based hard-rock style, Aerosmith was the top American heavy-metal band of the mid-Seventies".