1. Tall, narrow flanking-window or wing-light on either side of a wider door or window, often found in late-C18 and early C19 British houses of the grander sort (see fanlight).
2. Narrow lights defined by glazing-bars around the edges of a sash-window, often with coloured glass, and common in C19 Greek Revival architecture or that influenced by a taste for a wider, squatter proportion of rectangular window that became fashionable c.1810.
"margin-light." A Dictionary of Architecture and Landscape Architecture. . Encyclopedia.com. (August 19, 2018). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/margin-light
"margin-light." A Dictionary of Architecture and Landscape Architecture. . Retrieved August 19, 2018 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/margin-light