A bi-metallic coin is one struck from two distinct metals or alloys joined as a single piece – typically a central disc of one metal set into an outer ring of another.
The Italian 500 lire of 1982 was the first bimetallic coin ever minted for general circulation, featuring an aluminum-bronze center in a stainless-steel ring. The technology was developed by the Italian Mint specifically to combat counterfeiting. The format subsequently spread worldwide and became especially prominent with the euro in 2002 – both the €1 and €2 coins use bi-metallic construction.
For collectors focused on pre-1945 material, bi-metallic coins are not relevant, as the format is a post-war development.