Here's a comprehensive definition:
Christian mission consists in sending and being sent across significant boundaries of human social experience to bear witness in word in deed to the reconciling action of God in Christ Jesus through the power of the Holy Spirit.
Here's a short definition:
Mission is ministry in the dimension of difference.
So we're on mission when we're reaching out beyond who and where we are as particular communities. The boundaries crossed may be around the corner or across the world. They may be social or economic, educational or professional, ethnic or linguistic, geographical or national, or some combination of these. So reaching an especially recession-hit group in your neighborhood is just as missional as planting churches in central Asia or supporting AIDS orphans in southern Africa. It’s the outreach that distinguishes mission from ministry.
A particular gift of the Christian gospel is that it speaks both to and through every human culture. As God took flesh in Jesus, so the gospel is expressed through the world views and life-ways of people groups. This fungibility of the gospel means that when it is received by a people group, the gospel is also elaborated and illuminated in new ways. This has led to the extraordinary growth and diversity of Christianity, now with the most adherents of any religion in the world.
So world mission becomes an experience of discovering as well as sharing. Christians share the good news of what God has done in Christ through proclamation, forming new communities, and expressing God's reign in Jubilee justice and compassion. In a joyful return movement, Christians sharing the good news find that they discover what the gospel really is as it receives different and enlightening expression through the lives of people in different cultures. Today most missionaries testify that they discover more than they thought they knew and receive more than they give.
Receiving as well as giving in mission applies to all dimensions of mission work. Even in the strong liturgical tradition of Anglicanism, we find a rich and startling variety of worship traditions and styles in churches of the Anglican Communion around the world. Preaching and pastoral care are offered in ways from which we have much to learn. Evangelism with people of other faiths prompts the evangelist to fruitful reflection about the gifts of other religions and the contribution of one's own faith. Different churches' engagement with injustice and oppression in their contexts has much to offer to our work in our own context. In this way the mission journey becomes a pilgrimage. We discover more and more of who God is and what God is doing.
In this mutual gift exchange, companionship comes to the fore as a paradigm for mission in the 21st century. Companions walk together and share bread on the journey – the bread of the eucharist, the bread of discovering and celebrating the gospel, the bread of being in solidarity with one another in the glory and suffering of being the body of Christ in the world. The mission companion ministers as a witness, a pilgrim, a servant, a prophet, an ambassador, a host, and a sacrament. The mission companion is a sacrament of Christ's solidarity with the human community.
The mission ethos of companionship has the capacity to heal wounds that continue to fester from the alliance that mission has sometimes formed with imperial projects of racist oppression and economic exploitation. Companionship has the capacity to bring healing in challenges of our time – widening disparities between rich and poor, intensifying inter-religious conflict, unequal effects of global recession, persistent gender and ethnic violence.
The substance of Jesus' mission was reconciliation. Reconciliation must the substance of Christian mission in the 21st century.