Answer: Muntin.
This answer comes by way of the Preservation Principles PDF from Newport, RI. Wikipedia has the following:
A muntin in US usage is a strip of wood or metal separating and holding panes of glass in a window.[1]Muntins are also called "muntin bars", "glazing bars", or "sash bars". Muntins can be found in doors, windows and furniture, typically in western styles of architecture. Muntins divide a single window sash or casement into a grid system of small panes of glass, called "lights" or "lites".
I thought they were called "Mullions". But I find that many of the things I can look up today, which were not as easy to look up in years past have turned out to be wrong.
Muntins are often confused with "mullions" (which separate complete window units)
Photo Credit: Matěj Baťha - Own work, CC BY-SA 3.0