There is a terminal window, owned by the OS. It has a minimize and a close button, for example.
Avi has an editor viewport, and this should take up the whole terminal window. (Someday it'll have tabs and panes.)
A document is the analog of a buffer in Vim; it contains the name and contents of a document.
A pane is a rectangular area of the screen. A lens connects a pane to a document (the lens tracks the cursor, the offset into the document, and other kinds of presentation concerns). So every pane has a lens, but a document can have more than one lens.
Also inside the editor viewport, below any panes, is a status line. That's where you probably see the filename you're editing in reverse text, when you're working in Vim.
Also inside the editor viewport, below the status line, is the command line. That's where things like :q or :sp README.md will appear.