A sermon is an oration or lecture by a preacher (who is usually a member of clergy). Sermons address a scriptural, theological, or moral topic, usually expounding on a type of belief, law, or behavior within both past and present contexts. Elements of the sermon often include exposition, exhortation, and practical application. The act of delivering a sermon is called preaching.
In Christian churches, a sermon is usually preached in a place of worship, either from an elevated architectural feature, known as a pulpit or an ambo, or from behind a lectern. The word sermon comes from a Middle English word which was derived from Old French, which in turn originates from the Latin word serm? meaning "discourse". A sermonette is a short sermon (usually associated with television broadcasting, as stations would present a sermonette before signing off for the night).
The Bible contains many speeches without interlocution, which some take to be sermons: Moses in Deuteronomy 1-33; Jesus' sermon on the mount in Matthew 5-7 (though the gospel writers do not specifically call it a sermon; the popular descriptor for Christ's speech there came much later); and Peter after Pentecost in Acts 2:14-40 (though this speech was delivered to nonbelievers and as such is not quite parallel to the popular definition of a sermon).
In Christianity, a sermon is typically identified as an address or discourse delivered to an assembly of Christians, typically containing theological or moral instruction. The sermon by Christian orators was partly based on the tradition of public lectures by classical orators. Although it is often called a homily, the original distinction between a sermon and a homily was that a sermon was delivered by a clergyman (licensed preacher) while a homily was read from a printed copy by a layman. In the 20th century the distinction has become one of the sermon being likely to be longer, have more structure, and contain more theological content. Homilies are usually considered to be a type of sermon, usually narrative or biographical, see sermon types below.
The word "sermon" is used to describe many famous moments in Christian (and Jewish) history. The most famous example is the Sermon on the Mount by Jesus of Nazareth. This address was given around 30 AD, and is recounted in the Gospel of Matthew (5:1-7:29, including introductory and concluding material) as being delivered on a mount on the north end of the Sea of Galilee, near Capernaum. It is also contained in some of the other gospel narratives.
During the later history of Christianity, several figures became known for their addresses that later became regarded as sermons. Examples in the early church include Peter (see especially Acts 2:14b-36), Stephen (see Acts 7:1b-53), Tertullian and John Chrysostom. These addresses were used to spread Christianity across Europe and Asia Minor, and as such are not sermons in the modern sense, but evangelistic messages.
The sermon has been an important part of Christian services since Early Christianity, and remains prominent in both Roman Catholicism and Protestantism. Lay preachers sometimes figure in these traditions of worship, for example the Methodist local preachers, but in general preaching has usually been a function of the clergy. The Dominican Order is officially known as the Order of Preachers (Ordo Praedicatorum in Latin); friars of this order were trained to publicly preach in vernacular languages, and the order was created by Saint Dominic to preach to the Cathars of southern France in the early 13th century. The Franciscans are another important preaching order; Travelling preachers, usually friars, were an important feature of late medieval Catholicism. In 1448 the church authorities seated at Angers prohibited open-air preaching in France. If a sermon is delivered during the Mass it comes after the Gospel is sung or read. If it is delivered by the priest or bishop that offers the Mass then he removes his maniple, and in some cases his chasuble, because the sermon is not part of the Mass. A bishop preaches his sermon wearing his mitre while seated whereas a priest, or on rare occasions a deacon, preaches standing and wearing his biretta.
In most denominations, modern preaching is kept below forty minutes, but historic preachers of all denominations could at times speak for several hours, and use techniques of rhetoric and theatre that are today somewhat out of fashion in mainline churches.
During the Middle Ages, sermons inspired the beginnings of new religious institutes (e.g., Saint Dominic and Francis of Assisi). Pope Urban II began the First Crusade in November 1095 at the Council of Clermont, France, when he exhorted French knights to retake the Holy Land.
The academic study of sermons, the analysis and classification of their preparation, composition and delivery, is called homiletics.
A controversial issue that aroused strong feelings in Early Modern Britain was whether sermons should be read from a fully prepared text, or extemporized, perhaps from some notes. Many sermons have been written down, collected and published; published sermons were a major and profitable literary form, and category of books in the book trade, from at least the Late Antique Church to about the late 19th century. Many clergymen openly recycled large chunks of published sermons in their own preaching. Such sermons include John Wesley's 53 Standard Sermons, John Chrysostom's Homily on the Resurrection (preached every Easter in Orthodox churches) and Gregory Nazianzus' homily "On the Theophany, or Birthday of Christ" (preached every Christmas in Orthodox churches). The 80 sermons in German of the Dominican Johannes Tauler (1300-1361) were read for centuries after his death. Martin Luther published his sermons (Hauspostille) on the Sunday lessons for the edification of readers. This tradition was continued by Chemnitz and Arndt and others into the following centuries--for example CH Spurgeon's stenographed sermons, The Metropolitan Tabernacle Pulpit. The widow of John Tillotson (1630-1694), Archbishop of Canterbury received £2,500 for the manuscripts of his sermons, a very large sum.
The Reformation led to Protestant sermons, many of which defended the schism with the Roman Catholic Church and explained beliefs about the Bible, theology, and devotion. The distinctive doctrines of Protestantism held that salvation was by faith alone, and convincing people to believe the Gospel and place trust in God for their salvation through Jesus Christ was the decisive step in salvation.
In many Protestant churches, the sermon came to replace the Eucharist as the central act of Christian worship (although some Protestants such as Lutherans give equal time to a sermon and the Eucharist in their Divine Service). While Luther retained the use of the lectionary for selecting texts for preaching, the Swiss Reformers, such as Ulrich Zwingli, Johannes Oecolampadius, and John Calvin, notably returned to the patristic model of preaching through books of the Bible. The goal of Protestant worship, as conditioned by these reforms, was above all to offer glory to God for the gift of grace in Jesus Christ, to rouse the congregation to a deeper faith, and to inspire them to practice works of love for the benefit of the neighbor, rather than carry on with potentially empty rituals.
One early female writer of sermons in England was Mary Deverell (1731-1805).
In the 18th and 19th centuries during the Great Awakening, major (evangelistic) sermons were made at revivals, which were especially popular in the United States. These sermons were noted for their "fire-and-brimstone" message, typified by Jonathan Edwards' famous "Sinners in the Hands of an Angry God" speech. In these sermons the wrath of God was intended to be made evident. Edwards also preached on Religious Affections, which discussed the divided Christian world.
In Evangelical Christianity, the sermon is often called the "message". It occupies an important place in Worship service, half the time, about 45 to 60 minutes.  This message can be supported by a powerpoint, images and videos.  In some churches, messages are grouped into thematic series.  The one who brings the message is usually a pastor formed in a bible college.  Evangelical sermons are broadcast on the radio, on television channels (televangelism), on the Internet, on web portals, on the website of the churches  and through social media like YouTube and Facebook. 
Rabbinic ordination often includes the phrase, Rabbi, Teacher, and Preacher in Israel, and there is a long history of using sermons in Judaism as part of education, ethics, a call to repentance, or as a message of hope, often during difficult times.
In 1939, Rabbi Philip R. Alstat, an early leader of Conservative Judaism, spoke and wrote about the lesson of hope that the festival of Passover could give to the Jewish people, despite the rising power of Nazism in Europe: he counseled hope, and even gratitude, as part of Jewish strength to withstand the pain of events in Europe:
Perhaps in our generation the counsel of our Talmudic sages may seem superfluous, for today the story of our enslavement in Egypt is kept alive not only by ritualistic symbolism, but even more so by tragic realism. We are the contemporaries and witnesses of its daily re-enactment. Are not our hapless brethren in the German Reich eating "the bread of affliction"? Are not their lives embittered by complete disenfranchisement and forced labor? Are they not lashed mercilessly by brutal taskmasters behind the walls of concentration camps? Are not many of their men-folk being murdered in cold blood? Is not the ruthlessness of the Egyptian Pharaoh surpassed by the sadism of the Nazi dictators?
And yet, even in this hour of disaster and degradation, it is still helpful to "visualize oneself among those who had gone forth out of Egypt." It gives stability and equilibrium to the spirit. Only our estranged kinsmen, the assimilated, and the de-Judaized, go to pieces under the impact of the blow....But those who visualize themselves among the groups who have gone forth from the successive Egypts in our history never lose their sense of perspective, nor are they overwhelmed by confusion and despair.... It is this faith, born of racial experience and wisdom, which gives the oppressed the strength to outlive the oppressors and to endure until the day of ultimate triumph when we shall "be brought forth from bondage unto freedom, from sorrow unto joy, from mourning unto festivity, from darkness unto great light, and from servitude unto redemption.
In the same way, he preached a message of hope in 1938 when he said that, "Undaunted, we confidently expect that some day, somehow, the present low ebb of liberty and democracy will be followed by a rising tide whose onrush will irresistibly wash away the ramparts of tyranny." His sermons and articles targeted the Jewish community, the United States, the "family of nations," the "Jewish homeland in Palestine," and frequently described the importance of the "Jewish State"--a nation yet not created, but which he supported with both his words and his actions. He shared his vision of that State by proclaiming that, "Whether the Jewish State be large or small, its importance in the family of nations will be determined, not by its limited area, but by its creative genius and cultural contributions to mankind. Like Judaea and Athens of old, it may be only a small vessel, but exceedingly rich in precious content."
In societies or communities with (for example) low literacy rates, strong habits of communal worship, and/or limited mass-media, the preaching of sermons throughout networks of congregations can have important informative and prescriptive propaganda functions for both civil and religious authorities - which may regulate the manner, frequency, licensing, personnel and content of preaching accordingly.
There are a number of different types of sermons, that differ both in their subject matter and by their intended audience, and accordingly not every preacher is equally well-versed in every type. The types of sermons are:
Sermons can be both written and spoken out loud.
Sermons also differ in the amount of time and effort used to prepare them.
With the advent of reception theory, researchers also became aware that how sermons are listened to affects their meaning as much as how they are delivered. The expectations of the congregation, their prior experience of listening to oral texts, their level of scriptural education, and the relative social positions--often reflected in the physical arrangement--of sermon-goers vis-a-vis the preacher are part of the meaning of the sermon.
Albert Raboteau describes a common style of Black preaching first developed in America in the early 19th century, and common throughout the 20th and into the 21st centuries:
Historically, the American sermon has been one of the most vital forms of mass media. Few aspects of society have remained outside its purview and regulation.
[...] the most important of the homilies for our purposes is the tenth, 'An Exhortacion concerning Good Ordre and Obedience to Rulers and Magistrates'. It may have been written by Cranmer himself, although we cannot be sure. The sermon is proof that Tudor royal propaganda was directed at a mass audience.
The decrees of the Council of Trent that have to do with preaching spend a great deal of effort on regulation, stipulating where and when preaching has to occur, who is allowed to preach, how the vocation to be a preacher works, and so on. Episcopal oversight over preaching is particularly precisely regulated. Behind this juridicial regulation lies the attempt to avoid, under all circumstances, the penetration of Protestant preachers into Roman Catholic congregations.
The volume concludes with three appendixes of primary sources to aid understanding of the theories, reception, and regulation of preaching. The third of these ('Preaching Regulated') assembles in one place for the first time all the official acts and proclamations that governed preaching in England, Scotland and Ireland from the Reformation to the late seventeenth century.
[General Alamsjah,] the first Minister of Religious Affairs to develop the model of religious harmony in practice [...] developed a variety of policies increasingly instrusive in nature. [...] [T]he regime regulated how the kuliah subuh (sermon following the dawn prayer) should be presented through radio broadcasts.[...] It also made rules on the allowable terms, methods and contents of dakwah in sermons to audiences.[...] Moreover, certain technicalities on delivering dakwah or preaching were also tightly regulated. For example, the instructions of the Directorate-General of Islamic Guidance contained guidelines for the use of loudspeakers in mosques, and other smaller Islamic places of worship like mushalla and langgar.
The dictionary definition of preach at Wiktionary