Preaching is verbal oral communication of which the following things are true:
It’s contents is God’s gracious and special self-disclosure. For evangelicals, its contents is the Bible, as it’s focus is Jesus Christ.
It is biblical truth mediated through human personality.
Its immediate purpose is to inform, persuade, appeal, invite a response, encourage, rebuke, instruct in righteousness. More generally put: to illicit an appropriate human response to the God who’s revelation is its content.
Its ultimate goal is the glory of God and the calling forth and edification of the church.
It has an inescapable heraldic element.