On the desk in his office, the pastor of our church has this definition of expository preaching, which he ascribes to someone named Dr. Olsen. I'm not sure who that is, but I think this is a helpful working definition:

In recognition of the power of the Holy Spirit through the preaching of the Word of God, and expository sermon explains a given passage of Scripture clearly, interprets it carefully in its context and in relation to the whole of Scripture, and applies it relevantly by deriving its central idea and structure from that text in expectation of the response of the hearers in faith, obedience and worship.