This was mostly a thing in the PS1 and PS2 eras. The first genre was the point and click PC adventure game, ie the thing we still think of when we hear the words "adventure game" to this day. The second definition was much less well defined and encompassed games like Tomb Raider, Shadow of the Colossus, the Ubisoft Prince of Persia games and Soul Reaver.
I think it was just used as a catch-all term for any kind of console third person 3D action game with platforming elements that couldn't be easily classed into other genres
The idea of a "platformer" game doesn't really exist anymore, outside the niche pixel side-scrollers on Steam. A lot of the old "action/adventure" hybrids like Metroid and Mario and Zelda have been replaced by over-the-shoulder open world games and shooters. Hell, Metroid probably would have been shooters at inception, if the tech had been up to speed. Games like Contra tried to be on certain levels. Meanwhile, Zelda and Mario have really come into their own after M64 and Ocarina thanks to the open world style of play.
The modern idea of "adventure" is more intended to describe an exploration game with RPG elements, rather than a story-driven platform hopper. I think that's probably truer to the intent of the original game concept that "adventure" games were intended to describe.
“Platformers don’t exist outside of niche steam games”, said Elephant Mario as he slowly faded into nothingness, while a stream of tears ran down his face. A moment later the trombones finally stopped playing. He was gone.
It's amazing to me how a lot of games are still trying to chase Resident Evil 4. A game that's nearly 20 years old.