I claim the ornamental design for a medallion, as shown and described.
FIG. 1 is a front elevational view of a medallion, showing my new design;
FIG. 2 is a perspective view thereof; and,
FIG. 3 is a top plan view thereof, the bottom plan view, left-side elevational view, and right-side elevational view all being identical.
The broken lines showing a Roman numeral in FIGS. 1 and 2 depict environmental matter and form no part of the claimed design. The rear view forms no part of the claimed design.