The Chalcedonian Definition

The first thing one should notice from the title of this post is that the document produced at the Council of Chalcedon in October 451 was not a “creed”; it was a “definition.” 
A creed, properly speaking, is not a statement of what Christians believe about our faith. (That would be a “confession.”) Instead, a creed is a pledge of allegiance to the Father, the Son, and the Holy Spirit. Creeds answer the question, “In whom do you believe?” more than the question “What do you believe?”
Creeds were originally intended for liturgical use, as the people of God affirmed their allegiance to the persons of the Trinity prior to baptism or the celebration of the Eucharist. In contrast, a definition is a commentary on a creed, designed to give more terminological precision to the content of that creed.
The Council of ChalcedonAt the Council of Chalcedon (the Fourth Ecumenical Council in the Greco-Roman world), the bishops who assembled were firmly convinced that the Nicene Creed was sufficient to affirm their faith in God, his Son, and his Spirit.
A definition is a commentary on a creed, designed to give more terminological precision to the content of that creed.Click To TweetThey were right: ​

The Chalcedonian Definition - Credo Magazine
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