It's a modern tag used to describe the sub-genre of heavy metal more akin to the sound of Judas Priest, Motorhead, Thin Lizzy, Black Sabbath, Iron Maiden (and the whole NWOBHM sometimes) specially towards the 80's.
It's a classification that defies that a band can be tag along power metal (like Hammerfall), doom (like Candlemass), thrash (like Coroner), prog (like Dream Theater), etc.
By my standards if you check bands like Metalucifer, Hammerheart, Wolf, White Wizzard more or less they are around that definition.