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