In an excellent article in Leadership Magazine, Alan Hirsch sets out his view of what the word missional means. I find it hard to pick fault with him.
A proper understanding of missional begins with recovering a missionary understanding of God. By his very nature God is a “sent one” who takes the initiative to redeem his creation. This doctrine, known as missio Dei—the sending of God—is causing many to redefine their understanding of the church. Because we are the “sent” people of God, the church is the instrument of God’s mission in the world. As things stand, many people see it the other way around. They believe mission is an instrument of the church; a means by which the church is grown. Although we frequently say “the church has a mission,” according to missional theology a more correct statement would be “the mission has a church.” (read more).
There is also an interesting tree diagram showing the history of missional publications over the last ten years. I heartily endorse his praise for Guder’s Missional Church: The People of God Sent on a Mission (Gospel & Our Culture)