A sermon, in the broadest sense used here, is a delivered address on religious teaching given by an authorized speaker to a gathered community, most often inside a scheduled service rather than as free conversation. Three traditions build a version of this practice from very different starting points, and comparing them says more about what a tradition asks of its officiants than about a shared origin.
The Christian sermon or homily developed out of the ancient synagogue custom of expounding a scripture reading after it was recited, a pattern the earliest Christian communities inherited directly and then extended into a fixed feature of the Sunday service. Its content is normally tied to a reading assigned for that day, and who may preach it is governed by ordination rules that differ sharply between traditions.
The Islamic khutbah, delivered by a khatib before the Friday congregational prayer, is not merely customary but a required element of that prayer in the major legal schools, with its own historical weight as a vehicle for political and legal authority as well as religious instruction. A ruler or governor named in the khutbah was, for centuries, making a public claim to legitimate rule.
The Buddhist dharma talk shares the basic shape, a teacher addressing an assembled audience, but answers to no comparable requirement. It is not tied to a fixed liturgical reading and no single schedule governs when one must be given, and a monastic community may hold one in response to a question rather than on a set calendar.
The three are related by function, not by descent. This entry treats them as three named practices under one descriptive heading rather than as one practice with three names.