It is actually neither. Dinner was what one had before evening tea, between lunch and supper. Dinner was a meal that consisted of fats and carbohydrates with proteins that needed to be digested before bed time so that the people could maintain healthy lifestyles. Proteins take several hours to digest. Your body enters a rest state when you sleep that makes digesting a lot of food difficult. Remember that portions back then for any meal would be smaller than what we feed our ten-year-olds today.
Supper was a light meal with cooked vegetables and often a broth or soup (hence the name) and this would be to aid in the digestion of the previous dinner.
Lunch in many places is merely a snack, and breakfast is a large meal containing everything Americans would normally have for dinner (meat, carb, veggies, fruit, occasionally ale or wine). Lunch was designed to carry you over to dinner, which came at the end of the work day.
The use of a dinner faded when technology advanced, from industry to crop growing techniques, and most societies developed a three meal, rather than a four meal, day.
The best caricature of dinner is expressed by J.R.R. Tolkien's hobbits.