Definitions

1. noun

A vertical bar or division between the lights of a window, typically made of stone or wood.

“The Gothic cathedral’s elaborate stone carvings included intricate mullions and tracery.”

2. noun

A vertical structural member in a wall, typically dividing a window or opening into smaller parts.

“The building’s architect specified the use of mullions to add visual interest to the facade.”

3. verb

To divide or separate something, typically into smaller parts, by vertical bars or structural members.

“The designer chose to mullion the large window to create a sense of separation between the living and dining areas.”