mullion

A vertical member, as of stone or wood, dividing a window or other opening.

To form into divisions by the use of mullions.

In architecture:

One of the divisions between panels in wainscoting. Formerly monial.

A slender bar or pier which forms the division between the lights of windows, screens, etc.

An upright member of a framing. See stile.

To furnish with mullions; to divide by mullions.

A vertical bar between the panes of glass or casements of a window or the panels of a screen.

a nonstructural vertical strip between the casements or panes of a window (or the panels of a screen)