What is the rarest checkmate?

Could you elaborate on the concept of "the rarest checkmate" in the context of chess? Is it referring to a unique or highly unusual sequence of moves leading to checkmate? Or is it a reference to a specific chess piece or board configuration that is incredibly difficult to achieve? Furthermore, are we discussing this in the realm of traditional chess or perhaps a variant that introduces new rules or pieces? Understanding these nuances will help frame a more accurate and informative answer.