"Missio Dei" is a Christian/Latin term meaning "the sending of God" or "the mission of God" (I like "sending" better). Preaching on that this Sunday at QC (Quest Church). The term was coined by Karl Hartenstein in response to Karl Barth and his emphasis on "actio dei" ("the action of God") --- all of this according to wikepedia.com.
In 21st century American church culture, we tend to speak of "mission" as something we "do" (example - we have "Missions Committees" or we go on "Missions trips", etc.). However, the Biblical concept of church and mission has much more to do with WHO WE "ARE" ... not in what we "DO." The term "missio dei" places emphasis on the reality that mission is born out of the very nature of God. It is WHO HE IS. And because of WHO HE IS, God acts be "sending" (Missio). Think about it - God SENT His Son ("missio dei"). And now because of this, Jesus invites the Church to participate in the "missio dei" --- we are invited to become part of the mission of God in the world we live in. In this sense, Church IS a mission. We don't DO missions. We are participating in the very nature of God in the world. We are the hands and feet of Jesus to the world.
This is revolutionary for the modern American church. But if we are to become the Church we are called to be ... we must seek what God would make us... and not what we want to simply become on our own. I pray that we would be SENT to participate in the "missio dei" in the communities we live in!