The belief that two (or more) substances exist in reality. A sensible person cannot deny that the material world exists, so effectively this means that dualism is the belief that something immaterial exists in addition to the material world. If one believes in phenomenal consciousness, libertarian free will, or the soul, then this probably means that they believe in some form of dualism.
Does anything besides the material/physical world exist? Please let your voice be heard in the forum.