The issue of musical genres, subgenres, and micro-genres is decades old and, in my opinion, quite useful because it helps frame the music of someone you don’t know that well.
So, there are some basic considerations regarding this that I’d like to highlight. For instance, defining a band, a solo artist, or an album simply as "rock" is not very helpful (James Taylor and Sepultura are both "rock," for instance). It serves only to distinguish them from other unrelated genres... classical music or Al Bano or bossa nova, etc.
Moreover, the habit of categorizing all that vast post-Beatles Dylan Stones Who etc. popular music under the label of rock is not universal. In the United States, for example, they prefer to call it rock’n’roll, a term that here is associated with a narrow subgenre that includes Little Richard, Chuck Berry, etc., all the way to Little Tony and Bobby Solo.
So, all subgenres of our rock are welcome, among which metal is one of the main ones, and indeed, not by chance, in music stores and record stalls it often has its own specific section.
The real mess then, among all these rock subgenres and also metal ones, is the boundaries between one and the other. Because there are musical entities that perfectly fit the genre to which they belong, and others that instead oscillate between two or more genres. And here discussions can erupt... Is Zappa jazz or rock? What about Weather Report? Are King's X metal or progressive?
The solution to define these borderline people has been to associate two very different genres. Zappa and Weather are jazz rock (or fusion, even better). King's X are progressive metal.
And so, I consider Def Leppard to be pop metal. Catchy choruses and layers of harmonies over beautiful, strong bass and drum guitars. As the Count writes above, they earned their metal credentials from their NWOBHM origins but even then they were quite melodic.
I could place them in AOR, but isn’t adult rock an American thing? Boston and Toto and Foreigner? The Defs are heavier than AOR groups, but not by much... let’s say, for example, like Night Ranger, who, because they are Californian, have full rights to the AOR rank despite being even heavier, in terms of bass and drums, than Leppard.
It’s tough... in fact, it’s impossible to converge on the same genre definition for these borderline groups, but this is just to avoid getting stuck on just one of the two words that, in my opinion, define Def Leppard: they are not metal, but they are not pop either.
They are pop metal. They bring the grit of metal and the catchiness of pop.
The truth is that talking about musical genres associated with this and that is fun but pointless because everyone’s perspective starts from the type of music they love and primarily listen to.
In other words, for those who are crazy about Sepultura, Leppard are pop (sissies, as I wrote in my review). For those who go wild for Madonna, Leppard are metal (noise, always from my review).
For those who enjoy rock, hard rock, soft rock, progressive rock, and so on, in my opinion, there should be the definition of pop metal.
In the end, it doesn’t change anything, it’s all good, but it’s nice to keep each other company among enthusiasts, discussing with a fine balance between one definition and another.