word choice - "Lunch" vs. "dinner" vs. "supper" — times and meanings?

I've seen cases where a noon-time meal is referred to as dinner, and the evening meal is called supper. There's also lunch around noon followed by dinner in the evening. Is there a particular difference between dinner and supper, or a circumstance where lunch becomes dinner?

Answer

Dinner is considered to be the "main" or largest meal of the day. Whether it takes place at noon or in the evening is mostly a cultural thing. For instance, many people who grew up in the American South and/or on farms traditionally ate larger meals at noontime to give them the strength to keep working through the afternoon.

Supper is more specifically a lighter evening meal. Rooted in the word "to sup", it comes, again, from farming traditions — many farming families would have a pot of soup cooking throughout the day, and would eat it in the evening — specifically, they would "sup" the soup.

Lunch is almost the midday equivalent of supper — it's also a lighter and less formal meal than Dinner, but is used specifically when referring to a midday meal. So whether you use lunch/dinner or dinner/supper is heavily determined by when your culture traditionally has its largest meal.

Much Later Edit: I happened across this article discussing the agricultural roots of midday dinner and evening supper, just to add a bit more to the conversation.