1) A vertical bar which divides the lights in a window.
1538 ‘the chapel wants ... a mullion of stone’, Knaresborough
1663 Hen: Lawson pro stones for the mullions, 8s, Ripon. The earlier history of this word is complicated but it is considered to have developed by metathesis from ‘muniall’ which had the same meaning: 1379-80 Et in diversis monyeles lapid. esand et figand. cum plumbo in le Westgavell, Ripon.