Didn't want to highjack another post (Just What is Fly fishing?)

On the aforementioned thread there is a picture of a NICE trout taken byCO Flyfisher from the Niagara River. It got me to wondering:

What makes a steelhead a steelhead, instead of a lake rainbow running upstream? Here in CA, I grew up believing that a Steelie was a fish which went to sea, through the surf, therefore gaining a "head of steel" to get there and back. Any other rainbow was a rainbow.

Do the Great Lakes rainbows go up the St. Lawrence (or other) streams to the sea? If not, what makes a Steelhead?

Is there a minimum size to the lake? If you catch the fish in said lake, is it considered a steelhead? If I catch a 'bow in Lake Tahoe, Lake Superior, or Lake ________ is it considered a steelhead?

A minimum size to the fish? In California, a rainbow of 16" or greater, in a stream which is open to sea access (anadromous stream) no matter how far (or how many joinings with other waters) from the ocean, is considered a Steelhead. Silly, I know, but there it is.

I saw that the other thread about what is FFing stayed civil, and have seen other discussions of the same topic devolve into angst. I am hoping for the same, because I want to know what other people think.

Thanks
Mike