Yes… perhaps you're right, but you are being anal. Here's the description that tells the story. They're both part of windows. I will now refer to them as "Muntins".
"The whole shebang - sash, jambs, sill and everything else - is called a window. Mullion/muntin: A mullion is a heavy vertical or horizontal member betweenadjoining window units. Muntins are the narrow strips of wood that divide the individual panes of glass in a traditional sash."
