Well, to my knowledge, there is no word for a thing like that other than a "window frame on steroids".
I'm thinking that that window may have been installed after the house was built, and so it may have been necessary to support the ceiling joists over the window, and that's why that "window frame" is so robust.
What's under that thing? That is, what supports it?
Also, can you tell if that lumber is solid or it's hollow and just made of boards?