Answer by Franklin Veaux:
Dinner is the largest meal of the day. It's a term that indicates the size of the meal, not the time of the meal.
In some places where I've lived, like Florida and Oregon, it is customary to have the largest meal in the evening, so we talk about having breakfast, lunch, and dinner. When I lived in Nebraska, it was customary to have the largest meal at midday, so we would talk about breakfast, dinner, and supper.