Dinner is usually the name of the main meal of the day. Depending upon culture, dinner may be the second, third, or fourth meal of the day. It is still occasionally used for a noontime meal, if it is a large or main meal.
Dinner was the first meal of a two-meal day with the dinner heavy meal at noontime. The word is from the Old French disner, meaning "breakfast."
More meals were added and the morning meal became breakfast, because we 'break the fast' of not eating since the day before. Eventually, dinner shifted to referring to the heavy main meal of the day, even if it had been preceded by a breakfast meal. The (lighter) meal following dinner has traditionally been referred to as supper
Luncheon, commonly abbreviated to lunch, is a midday meal, and is generally smaller than dinner, which is the main meal of the day whenever dinner is eaten. The origin of the words lunch and luncheon relate to a small meal originally eaten at any time of the day or night, but during the 20th century gradually focused toward a small meal eaten at midday.
So, there it is - Breakfast, Lunch, Dinner for some. Others say it is Breakfast, Dinner, Supper. Still others say Breakfast, Lunch, Supper. None of these are to be confused with Brunch, which is a combo of the words breakfast and lunch. Snacks are not meals, so they contain no calories.