A muntin is a bar that separates two panes of glass within a window, helping to hold the window glass in place.
A mullion is a bar that separates units of windows.
A sash divider is an individual bar that divides panes of glass in a window. A sash divider is different than a muntin in that it is not physically dividing the glass, but is placed on top of the glass to create the appearance of a divided window.
A window grille (also called a window grid) is a collection of sash dividers that are bound together. Window grilles do not actually separate panes of window glass, but sit on top of one big window pane and create the appearance that the glass is divided into diamonds, rectangles, squares, or other shapes.
A window grate is a frame made up of intersecting parallel and vertical bars. It is usually made out of iron or other heavy metal material, and is often used for security purposes.
A divided light window is made up of several individual panes of glass that are held together by muntins.
Stimulated Divided Light (SDL):
A stimulated divided light window is made up of one pane of glass that has a window grille placed on top of it, creating the illusion of divided window panes.