The French word trumeau, originally referred to the space between windows. In the 18th century, the French started manufacturing trumeau mirrors to hang in these spaces, providing a decorative element and bringing more light into the room.
Antique trumeau mirrors range from carved, unpainted wood to painted and partially gilded, all the way to highly ornate and entirely gilded. The mirror is almost always rectangular and if old, most often the glass is tarnished with grey and black spots.
For more info on trumeau mirrors and how to make your own, read HERE.