It’s amazing that such a concise little word can have so many meanings. First, let’s address the design related ones 1) First and foremost, you’ll hear people refer to the window above a door or opening referred to as a transom or transom window. Historically, these windows were hinged to allow for air flow (pre-central air and heat, you see). 2) A transom can also just be a horizontal crosspiece – like a lintel – that sits above a door, in a window, or between the door and the window above it. 3.) The term can also describe a piece of fixed wood or ornamental work mounted above a door.
Now on to the non-design related definitions… 4) Quite simply, the horizontal part of a cross. (You’re going to impress some Christians with this one.) 5) The collective horizontal planks that create the stern (a.k.a. the back) portion of a boat. (And you’re going to impress some pirates with this one.)