1. Cross-bar or beam.
2. Horizontal element framed across a window, generally used during the Perpendicular period, and commonly found in the grid-tracery of Elizabethan and Jacobean houses, dividing the window-aperture into lights framed by bars forming the mullions and transoms.
3. Horizontal piece in a doorway forming part of the frame, above which is a fanlight.
"transom(e)." A Dictionary of Architecture and Landscape Architecture. . Encyclopedia.com. (April 17, 2019). https://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/transome
"transom(e)." A Dictionary of Architecture and Landscape Architecture. . Retrieved April 17, 2019 from Encyclopedia.com: https://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/transome