I would not provide a "definition", merely my opinion: a "heavy metal" album toned in a production type that made it fit to be playable in discotheques, and some songs even in the dinner party one's grandma gives in her place. I feel like giving a bit of perspective below.

To me, it has always seemed like a kind of a "follow the trend": a bit earlier, Smells Like Teen Spirits, a heavy song, was being played left and right in disco parties, fueling the dance of girls who would, not too long before that, puke at such an aggressive sound (and that was a good thing, mind you; made such parties bearable). I was 16 at the time and remember it vividly.

Interesting fact: had some Metallica-fanatic friends that would literally cry over the Black Album. The fact that Maiden had Wasting Love, a true metal ballad with a hard theme, did not soften the blow to most of them, since it was not the tonic of Fear of the Dark, the Album. They were destroyed by the loss of their favorite band to the mainstream.

Here are some other categories where Megadeth and Metallica are contenders for the top spot. And the awards go to:

Biggest Crybaby: Dave Mustaine
Biggest asshole: tie between Dave Mustaine and Lars Ulrich
Most anti-consumer: Metallica
Best lineup in the history of thrash: 90's Megadeth (Mustaine, Ellefson, Friedman, Menza)

Biggest difference between their best and worst music:  Metallica.  I think it's inarguable that Metallica has made the worst songs in the history of thrash. Although you could say a whole lot of it is not thrash at all.  Then they have three albums that are absolutely excellent. 

So it was a more commercial album that appealed to more people? Is that not what Megadeth spent most of the 90s also doing? I could be wrong, but wasn't Youthanasia a conscious attempt to try and write an album that could get to number one on the charts?

Say what, mate?

Cool pic. But Dave Mustaine is in the running for biggest cry baby to ever live. He's still crying about being kicked out of Metallica, 40 years later. 

lol please tell me you got the joke at least? 

Yeah, I got it.  

@bold: I wouldn't know about that. I am not that well versed in neither of the bands discography. As I said, to my knowledge, Deth had not produced a "Black Album" type of record, but I could be educated in that regard, even if I really don't care that much about either of them to be honest.

Now, directly to your point, I really do not think that a song like "À Tout le Mond" is in the same level of commercial cheese material as, say, "Unforgiven" and, mainly, "Nothing else Matters". It's just a more commercial, hard rock style song, than other things the band usually do that I would personally not compare to those other ones. And I think Dave knows (if he does not, he is plain stupid) that his voice could never carry them to a top of the pops position.

Bolded part - that's not what you said. You said "But if I had to choose, probably Megadeth, for the simple fact they never released a Black Album of sorts." Nothing about 'to my knowledge'. That's a pretty definitive statement implying you know both of their catalogues well, and saying that Megadeth never commercialised their sound or made more accessible music, which isn't true. They softened their sound gradually throughout the 90s. Listen to something like Wanderlust or Ecstasy off Risk - very accessible stuff.

Not that I'm really bothered either way, but I think your reasoning (at least as you originally expressed it) is flawed.

You didn't read my whole text, or chose to leave out a relevant part of it. Here is my direct quote: "Both are good bands technically but neither has a killer singer like Maiden has (different styles, I know, but it makes a great difference to me), so I never bothered too much since I would prefer the great vocalists..."

Like I said, I never bothered too much with neither, but that does not prevent me from saying Deth has never released a completely pop, purely commercial album as Metallica's "Black" one. I never heard anything like it from theirs, so, to me, it is indeed a hard never. No flaw in my thinking. Youthnasia is not similar to it, specially the kind of production behind it, but if you think it is, it is your opinion.