<p><strong>Noun</strong></p><ul><li>a vertical member, as of stone or wood, between the lights of a window, the panels in wainscoting, or the like</li><li>one of the radiating bars of a rose window or the like</li></ul><p><strong>Verb</strong></p><ul><li>(used with object) to furnish with, or to form into divisions by the use of, mullions</li></ul>