Some time ago I went through and made a stab at defining the Gospel. I thought I'd continue plowing through defining terms and throw out these definitions for 'missional':
A “missional” life is a Gospel-centered life in its external dimension. It is a Spirit empowered life lived in conformity to the person of Jesus Christ, who was sent by God to reconcile all things to Himself and to establish peace. A “missional” life participates in the ongoing work of renewal by the Holy Spirit by establishing a presence within and for the world that is faithful to Gospel, even while remaining distinct from the power structures of the world.
A “missional church” is a Gospel-centered church in its external dimension. As such, a missional church identifies itself as the church of Jesus Christ, who entered the world to reconcile it to God by living and dying for the world. Our missional activity is a response to the news of this reality, but it is a real participation in the restoration of the created order that was inaugurated by the resurrection of Jesus Christ and that continues through the empowering presence of the Holy Spirit.