Cheating is "act dishonestly or unfairly in order to gain an advantage."
You're not being dishonest or unfair when you mod or (use a) cheat in a single player game, you're free to do as you please, however if other players are involved, there are categories of mods that would fit that description.
You're not being dishonest or unfair when you mod or (use a) cheat in a single player game, you're free to do as you please, however if other players are involved, there are categories of mods that would fit that description.