Does "scrod" refer to one fish, multiple fish types, or what?

Scrod is usually defined as smaller pieces of cod or haddock – it appears to be some sort of a generic term referring to a couple of different types of fish (see Wikipedia for their definition). http://en.wikipedia.org/wiki/Scrod

However, I’ve seen “Boston Scrod” on the menu at seafood restaurants.

Is there a single scrod? Or are there many scrods? I need a scrod I can believe in. Can you northeasterners guide me?