Here are three approaches, each of which I find helpful in its way.

Austin Phelps

For the first, we go back to Austin Phelps (1820-1890).  A sermon, he says, is

1. an oral address.  It follows, or should follow, the rules of spoken, not of written communication.
2. an oral address to the popular mind.  It is distinct from, say, a scientific lecture, from a judicial oration, and from a harangue to a rabble.  It is addressed to the many, not to the few; to ordinary people, not to specialists.  It belongs to the real world, not just to the world of books.  This gives the sermon a higher, rather than a lower intellectual dignity.  ‘To make the deep thoughts of theology intelligible to all orders of mind, and impressive to them all, so that the same truth which instructs the ignorant, and quickens the torpid, shall also move the wisest, and command the most alert, is a masterly work of mind.’  Again: ‘Doddridge speaks with dolorous magnanimity of the effort which it cost him to discard from his style certain words, metaphors, constructions, which his literary taste tempted him to use, but which his conscience rejected as unsuited to the capacities of his hearers.  This was mourning the loss of useless tools.’
3. an oral address to the popular mind, upon religious truth.
4. an oral address to the popular mind, upon religious truth, as contained in the Christian Scriptures.
5. an oral address to the popular mind, upon religious truth as contained in the Scriptures, and elaborately treated.  A sermon is not a series of informal or spontaneous remarks.  It is constructed with thoughtfulness and care.
6. an oral address to the popular mind, upon religious truth contained in the Scriptures, and elaborately treated with a view to persuasion.  Its aim is not to entertain, like a theatrical performance, nor merely to be beautiful, like a poem, nor even to be instructive, like a lecture.

Phelps, Theory of Preaching, Lectures 1 & 2.

J. I. Packer

The second approach is that of J.I. Packer, who suggests that preaching can be defined in a number of ways.  It can be defined institutionally, presenting preaching in terms of buildings, pulpits and pews.  According to this definition, preaching is what a man (or woman) does when he (or she) ascends the pulpit steps on a Sunday and delivers a discourse to a congregation within the context of an act of worship.  Preaching can be defined sociologically, viewing it as a specialised form of monologue fulfilling certain expectations on the part of the group being addressed.  But preaching can (and should) also be defined theologically and functionally.  Accordingly, preaching may be defined as

‘the event of God bringing to an audience a Bible-based, Christ-related, life-impacting message of instruction and direction from himself through the words of a spokesperson.’

This definition is (a) theological: its idea of preaching is not so much as human performance as divine communication; (b) prophetic: it views God as communicating his message via a messenger; (c) incarnational: God embodies his message in the messenger, both as he speaks it and models it; (d) critical, in that it provides us with a test of pulpit utterances.

J.I. Packer, Collected Shorter Writings, Vol 3, 277f.

David Day

The third approach is David Day’s.  Preaching, he writes, is

1. authoritative speech.  It is delivered in the name of God.  But there is a danger of it becoming authoritarian.
2. personally revealing speech.  “Preaching is truth embodied in personality” (Phillips Brooks).  The danger here is of egoism.  ‘The pulpit can license the most excruciatingly self-indulgent revelations.  Blissfully unaware of the effect he’s having on the embarrassed congregation, the preacher continues to spill out his entrails on the altar of self-disclosure.  The listeners squirm and pray that they might disappear down a crack between the flagstones.  And his teenage children contemplate patricide.’
3. crafted speech.  This takes time and effort.  A preacher was urged by a friend not to prepare his sermons, but to go up into the pulpit, relying on the Holy Spirit to speak to him.  The following week they met, and the preacher was asked if he tried out the idea.  “Yes.”  “And did the Holy Spirit speak to you?”  “Yes.  He said I had been very lazy.”  But, on the other hand, sound preaching method should never turn into manipulative technique.
4. persuasive speech.  The purpose of preaching is to change people.  Not every sermon can have unforgettably dramatic results.  Many will resemble regular breakfasts – essential, without being particularly memorable.

See Day, A Preaching Workbook, 6-9.