Yes, it's also an element, compound and alloy. Throughout its entries, Wiki draws a distinction between hard rock, heavy rock and heavy metal. The heavy metal definition implies that, at least in the early stages, it was/is an alternative name for heavy and hard rock, with which I concur. I disagree with them that it necessarily has a thick heavy sound, as one of their own examples, Led Zeppelin, do not. Wiki go on to say that Judus Priest evolved heavy metal by dispelling the blues element and that the NWOBHM bands such as Iron Maiden added a punk sensibilityand speed. The dispelling of the blues is arguable, 'punk sensibility' is a woolly and meaningless term and many of the original hard rock bands were fast anyway. The Oxford Dictionary gives the definition of 'heavy metal' as 'hard rock', pure and simple! The most hard rock? Black Sabbath, closely followed by Budgie!