I tend to define it by the type of meal I'm having.
Breakfast is breakfast. That's not going to change. Though maybe if I was a shift worker it might change my definition of the time I eat that particular meal.
If I'm having a light meal around mid day, then that's lunch. If I'm having my main meal then, it's dinner.
If I'm having my main meal in the evening, that's dinner, but if I'm having a snacky type meal in the evening, then it's tea (or a bloody poor excuse for a dinner lol).