Definition: A muntin is a strip (either real or illusory) separating panes of glass. Because it was impossible or prohibitive to manufacture large sheets of glass prior to the 20th century, big expanses of windows were achieved by holding many, smaller panes of glass together by muntins.
A true muntin is a strip of wood or similar material completely separating panes of glass. In the modern world, the illusion of muntins is created by sandwiching thin strips of aluminum or plastic between double-paned glass, or by affixing these grilles onto the outside of the glass.
A mullion is not a muntin. A mullion separates window units, not panes of glass.
Wood muntins held the 8 panes of glass in place to create a single window.