"Challenge coin" is a term that's new to me.
Art
google it
It refers to the commemorative coins military and other folks tote around that have a unit emblem or such-like on them. In theory, if you are out and someone tosses one on the table you have to produce your own, the guy without has to buy the round.
A 1/8" thick coin of 1-1/2" diameter, most commonly bronze but available in precious metals as well.
