1
b
: a horizontal crossbar in a window, over a door, or between a door and a window or fanlight above it
c
: the horizontal bar or member of a cross or gallows
d
: any of several transverse timbers or beams secured to the sternpost of a boat
also
: the planking forming the stern of a square-ended boat
2
or transom window
: a window above a door or other window built on and commonly hinged to a transom
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Merriam-Webster unabridged