You're very own source:
http://www.thefreedictionary.com/dinner
Obviously supper only refers to a evening meal, not arguing that. I don't understand why people are so insistent that dinner cannot though. I'm merely pointing out that by the use of "lunch" and "dinner" at restaurants, the latter is clearly more popular and understandably used to refer to an evening meal...even though it could refer to either one.
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