What is a sandwich? What is not a sandwich?

The current top hot item on /r/eatsandwiches is a post titled Are bacon cheeseburgers considered sandwiches? (General consensus is "yes," FWIW)

I pondered a similar question myself last night while eating homemade soft tacos. Are tacos sandwiches? A tortilla is a type of bread, is it not? And if the requirement were two slices of "tortilla", then open-faced turkey sandwiches wouldn't meet the criteria, but some quesadillas might.

Is a calzone a sandwich? What about an empanada? When I pile Ethiopian wat (stewed meat) into injera (traditional Ethiopian flatbread) am I making a sandwich? The list could go on and on!

Noble Redittors of Reddit, this important question must be debated and resolved forthwith!

What are the defining characteristics of the category known as "sandwiches" and what foods or dishes do and do not fit into that category?