Erik Raymond has helpfully collected a range of contemporary definitions:

John MacArthur: ‘The message finds its sole source in Scripture. The message is extracted from Scripture through careful exegesis. The message preparation correctly interprets Scripture in its normal sense and its context. The message clearly explains the original God-intended meaning of Scripture. The message applies the Scriptural meaning for today.’ (Preaching)

Bryan Chappell: ‘The main idea of an expository sermon the topic, the divisions of that idea, main points, and the development of those divisions, all come from truths the text itself contains. No significant portions of the text is ignored. In other words, expositors willingly stay within the boundaries of the text and do not leave until they have surveyed its entirety with its hearers.’ (Christ-Centered Preaching)

John Stott: Exposition refers to the content of the sermon (biblical truth) rather than its style (a running commentary). To expound Scripture is to bring out of the text what is there and expose it to view. The expositor opens what appears to be closed, makes plain what is obscure, unravels what is knotted, and unfolds what is tightly packed. (Between Two Worlds) [Published as ‘I Believe in Preaching’ in the UK]

Alistair Begg: ‘Unfolding the text of Scripture in such a way that makes contact with the listeners world while exalting Christ and confronting them with the need for action.’ (Preaching for God’s Glory)

Haddon Robinson: ‘The communication of a biblical concept derived from and transmitted through a historical-grammatical and literary study of a passage in its context, which the Holy Spirit first applies to the personality and experience of the preacher then through him to hearers.’ (Biblical Preaching)

Martyn Lloyd-Jones: ‘Preaching is theology coming through a man who is on fire and that the chief end of preaching is to give men and women a sense of God and his presence.’ (Preaching and Preachers) [This, be it noted, is not a definition of expository preaching!  For a something nearer to a definition, see pp 82-84 of that book.]

David Helm: ‘Expositional preaching is empowered preaching that rightfully submits the shape and emphasis of the sermon to the shape and emphasis of a biblical text.’ (Expositional Preaching)

John Piper: ‘Expository exultation.’ (The Supremacy of God in Preaching)

Albert Mohler: ‘Expository preaching is that mode of Christian preaching that takes as its central purpose the presentation and application of the text of the Bible . . . all other issues and concerns are subordinated to the central task of presenting the biblical text.’ (He Is Not Silent: Preaching in a Postmodern World)

Mark Dever: ‘Expositional preaching is preaching in which the main point of the biblical text being considered becomes the main point of the sermon being preached.’ (Preach: Theology Meets Practice)

Tim Keller– ‘Expository preaching grounds the message in the text so that all the sermon’s points are the points in the text, and it majors in the texts’ major ideas. It aligns the interpretation of the text with the doctrinal truths of the rest of the Bible (being sensitive to systematic theology). And it always situates the passage within the Bible’s narrative, showing how Christ is the final fulfillment of the text’s theme (being sensitive to biblical theology).’ (Preaching: Communicating Faith in an Age of Skepticism)

To Raymond’s collection of definitions I would like to add:

Philip Ryken – ‘Expository preaching is not so much a method as it is a mind-set. A minister who sees himself as an expositor knows that he is not the master of the Word, but its servant. He has no other ambition than to preach what the Scriptures actually teach. His aim is to be faithful to God’s Word so that his people can hear God’s voice. He himself is only God’s mouthpiece, speaking God’s message into the ears of God’s people, and thus into their minds and hearts.’

Peter Masters – Expository preaching is that which ‘draws the message from the biblical text, clearly and methodically, honouring the sense of the text, and the style of communication employed.’

James I. Packer – ‘Expository preaching is the preaching of the man who knows Holy Scripture to be the living Word of the living God, and who desires only that it should be free to speak its own message to sinful men and women; who therefore preaches from a text, and in preaching labors, as the Puritans would say, to “open” it, or, in Simeon’s phrase, to “bring out of the text what is there”; whose whole aim in preaching is to show his hearers what the text is saying to them about God and about themselves, and to lead them into what Barth called “the strange new world within the Bible” in order that they may be met by him who is the Lord of that world.’

Peter Adam – ‘Expository Preaching is the preaching of the message of a book of the Bible, usually verse by verse, paragraph by paragraph, chapter by chapter, by explanation and application of it to the congregation.’