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That's different for everyone. Just look what happened to the term rogue-like over the years.

And there is not even consensus what is a genre and what is merely a topic that appears in a game. Zombies are usually in horror games. But Plants vs. Zombies is hardly a horror game.

To me, Resident Evil is a horror game. And thus horror is a genre. But if I compare it to Slender Man, also a horror game, there are fundamental differences. 

So maybe it is, what is left when you take everything away from a game, until you cannot take the rest away and still call it that genre. For example, a platformer game without ability to jump or at least fall, is not a platformer to me. Does not have to be a single thing, could also be a combination of things.

Opinions differ about this, of course. You can go to Steam and look at popular games. The tags they have are the consensus of players, so they should at least give a glimpse of how most gamers would classify a game. Hence games being called rogue-like that have nothting at all to do with the game Rogue.