What Is a ‘Decoy’ in the Context of a Ring Signature?

A decoy, also known as a 'mixin,' is a public key belonging to another user on the blockchain that is included in a Ring Signature transaction. The decoy keys are mixed with the true spender's key to form the ring.

Their purpose is to mask the actual spending key, making the transaction appear equally likely to have been signed by any of the ring members. The selection of decoys is typically random from past transaction outputs.