It is a social thing.
For some groups, dinner is the main meal, taken at midday; for others it is a meal taken in the evening. A supper was a light meal taken before retiring to bed. It has now become interchangeable with dinner in the evening.
Lunch is also used for a main meal in the middle of the day. It derives from luncheon - a snack taken at any time in the day until late afternoon. As a word it has become restricted to very formal occasions.
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