Mullion
A vertical or horizontal structural member that divides and supports adjacent window units or glass panels within a curtain wall or storefront system.
Mullions provide the structural framework for curtain walls and storefront systems. In aluminum curtain wall systems, mullions are extruded aluminum profiles that accept glass panels, spandrel panels, and operable windows.
Vertical members are technically mullions while horizontal members are called transoms, though the term "mullion" is commonly used for both in the trade.