A transom is a horizontal bar or beam that divides a window or door, or a small window placed above a door. You've probably seen transom windows in older buildings: those rectangular windows sitting above doorways, often hinged at the bottom so they can tilt open for ventilation. Before air conditioning, people opened transom windows to let hot air escape from rooms while keeping doors closed for privacy.

On boats, a transom is the flat surface forming the stern (back end) of the vessel.

In everyday language, you might hear the phrase over the transom, meaning something that arrives unexpectedly or without formal introduction. Publishing companies used to receive unsolicited manuscripts over the transom, meaning authors would slip their stories through the open transom window above the office door. When someone says an opportunity came over the transom, they mean it appeared without anyone asking for it, like finding a surprise gift on your doorstep.