Here is a short definition of Biblical theology that I am working on, compiling and editing as I keep reading:
Biblical Theology is responding to the way the Bible is. Biblical Theology is responding to the fact that the Bible is an overall unity and structure. Biblical Theology sees each book of the Bible making its own contribution to the over all picture of God and his plan that climaxes in the person and work of Jesus Christ. Biblical Theology is trying to understand how every part of the bible relates to every other part and more specifically how every part of the Bible relates to Jesus Christ as the fulfillment of all God’s promises
see also www.disciplemaking.net