Speech, address, oration, harangue, lecture, talk, sermon, homily designate a discourse delivered to an audience.
Speech can apply to a public discourse irrespective of its quality or its degree of preparation, of its aim (as to influence, instruct, or entertain), or of the caliber of its speaker or audience.
Address implies formality and usually careful preparation; it often connotes distinction in the speaker or gives emphasis to the importance of the speech.
Oration suggests eloquence, rhetorical style, and usually a dignified but sometimes a high-flown or long-winded appeal to the emotions of a large audience or assembly.
Harangue, once nearly equivalent to oration except for its added implications of vehemence and passion, commonly retains only these distinctive implications and connotes either length and tediousness of speech or an impassioned appeal to the audience.
Lecture often implies reading; it commonly designates a carefully prepared speech on a special topic intended to give information and instruction to a group of students or studious persons.
Talk stresses informality; it may be used to designate either a lecture or an address when the speaker wishes to emphasize his desire to speak directly and simply to his auditors as individuals.
Sermon and homily both commonly imply religious instruction by an ordained preacher and a church congregation as the listeners; in such use sermon usually connotes a theme drawn from a scriptural text, while homily suggests practical moral counsel rather than doctrinal discussion. But both terms have extended use in which they denote a usually didactic talk or discussion on a moral theme.