Complex sentence: The Christological definition of Chalcedon states that Jesus was one person with two natures (the dyophysite position), in opposition to the Monophysite position that Jesus was one person with one nature.
Split into simpler sentences: The Christological definition of Chalcedon stated that Jesus was one person with two natures (the dyophysite position). This was stated in opposition to the Monophysite position that Jesus was one person with one nature.