The definition of a casserole is a food that is served in the same dish that it was prepared/cooked in. Like a macaroni and cheese casserole (not the box kind, the "real" kind- i.e. the hard kind no one ever makes). And I know that definition is right because Alton Brown wouldn't lie to me. He's so damn smart and dreamy.
I don't know what a hotdish is. I've never even heard the term. Is it regional?
I agree on the Alton Brown thing. My mom still makes mac'n'cheese in the oven like that for me [with homemade cheese sauce and breadcrumbs on top!] whenever I come home to visit. And hotdish is a midwestern term.
I think hotdish is an UPPER midwest term. I grew up in Ohio and Indiana and never heard that word until I moved to Minnesota, now I keep hearing it. At first I thought people meant hot dishes, like fresh from the dishwasher! LOL