I'm not so sure "heavy metal" has actually been defined and even if it was the definition would surely change over time. It's like "metal" fans keep trying to erase its history every few years and again redefine it as though doing so keeps it sounding "relevant" and "hip" Also many people have a narrow idea of what "metal" is because as soon as a band explores outside of the heavy metal box they get cast out which would make "heavy metal" very limited and one dimensional. And there's something I believe many have forgotten which is "metal" if indeed there is such a definite thing is a subgenre of rock n roll yet many "metalheads" have acted like it is its own entity while putting rock music in general down.
I've been on discussion boards before where even bands such as dio, priest, quiet riot, and the scorpions were questioned about whether they were metal or not One site even had a vote on bands about whether they were metal or not and those who were not considered metal in the poll was banned from being mentioned. I have a nephew for example who consideres maiden "rock" while calling motorhead metal. So yeah it has gotten absolutely ridiculous with bias and subjective ideas about what is metal or not and this is why I don't go on "heavy metal" boards any more. All this who's more metal or not is pathetic and I would rather not have any part of it.
The term "heavy metal" was coined as a loose term to describe zeppelins sound as being "like heavy metal falling from the sky" as far as I know, which was just to loosely describe the heavier or harder side of rock n roll and wasn't attended to be a label for a genre itself with one dimensional elements. So yes, at one time bands like ac/dc, rush, and even reo and journey were called "heavy metal" many times back in the 70's and even a part of the 80's. In the end it's all rock n roll.