Transom

Transom (probably a corruption of Latin transtrum, a thwart, in a boat; equivalents are French traverse, croisillon, German Losholz) is the architectural term given to the horizontal lintel or beam which is framed across a window, dividing it into stages or heights. In early Gothic ecclesiastical work transoms are only found in belfry[?] unglazed windows or spire lights, where they were deemed necessary to strengthen the mullions[?] in the absence of the iron stay[?] bars, which in glazed windows served a similar purpose. In domestic work, on account of the opening casements, they are more frequently found. In the later Gothic, and more especially the Perpendicular period, the introduction of transoms became very general in windows of all kinds.

The phrase "over the transom" refers to works submitted for publication without being solicited; the image being invoked that of a writer tossing a manuscript through the open window over the door of the publisher's office.