Breakfast is the first meal after you wake up, no matter what time it is.

Lunch is the meal that's not first, but no later than 4pm (or earlier than 10am)

dinner is a big lunch at the same time (like a lunch at church with the whole congregation, or at your grandpa's house with the family)

Supper is the evening meal ranging from 5-9pm

Dinner is a big supper at the same time (though usually on the later side) (like eating at a nice or moderately nice restuarant with the people you made all region with )

Anything else is snackage.

Basically breakfast is the only one that doesn't depend on time limits.