Here's my definition of cheating: The use of any 3rd party software, not allowed by the developer, that gives a material in-game advantage to the user against other players without their knowledge or consent.

So mods are not cheating. Purely single player hacks are not cheating. Optional 3rd party overlays or in-game aids like RuneLite for OSRS or the many external deck/stat trackers for games like Hearthstone or Genshin Impact also aren't cheating.

Using Aimbots, wall hacks, infinite health/ammo/lives, and similar in online comp play is cheating.

Grey zone stuff would be things like unapproved in-game overlays.

I don't care what people do in single player. Hack and mod as much as your heart desires, it doesn't hurt any other players. The worst thing that can happen is you "cheat" yourself out of a personal challenge, but that's your own call.