1. A round metal object, usually larger than a coin, cast or struck for commemorative purposes.

Medallion with portraits of Louis XII and Anne of Brittany. 1499. Gilded bronze.  Walters Art Museum, Baltimore.

2.  A round or oval frame containing a relief or pictorial image, used as decoration on a wall, a floor, or a piece of furniture or equipment.
Medallion with an allegorical  figure of Justice, Palazzo Montecitorio, Rome. Photo: Simone Ramella.