The Collaborative International Dictionary of English v.0.48:

Sermon \Ser"mon\, n. [OE. sermoun, sermun, F. sermon, fr. L.
   sermo, -onis, a speaking, discourse, probably fr. serer,
   sertum, to join, connect; hence, a connected speech. See
   Series.]
   1. A discourse or address; a talk; a writing; as, the sermons
      of Chaucer. [Obs.] --Chaucer.
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   2. Specifically, a discourse delivered in public, usually by
      a clergyman, for the purpose of religious instruction and
      grounded on some text or passage of Scripture.
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            This our life exempt from public haunts
            Finds tongues in trees, books in the running brooks,
            Sermons in stones and good in everything. --Shak.
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            His preaching much, but more his practice, wrought,
            A living sermon of the truths he taught. --Dryden.
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   3. Hence, a serious address; a lecture on one's conduct or
      duty; an exhortation or reproof; a homily; -- often in a
      depreciatory sense.
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The Collaborative International Dictionary of English v.0.48:

Sermon \Ser"mon\, v. i. [Cf. OF. sermoner, F. sermonner to
   lecture one.]
   To speak; to discourse; to compose or deliver a sermon.
   [Obs.] --Holinshed.
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         What needeth it to sermon of it more?    --Chaucer.
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The Collaborative International Dictionary of English v.0.48:

Sermon \Ser"mon\, v. t.
   1. To discourse to or of, as in a sermon. [Obs.] --Spenser.
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   2. To tutor; to lecture. [Poetic] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sermon
    n 1: an address of a religious nature (usually delivered during
         a church service) [syn: sermon, discourse, preaching]
    2: a moralistic rebuke; "your preaching is wasted on him" [syn:
       sermon, preaching]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "sermon":
   address, admonishment, admonition, assignment, castigation,
   chalk talk, chastisement, chiding, correction, discourse,
   disquisition, dressing-down, exercise, exhortation, exposition,
   harangue, homework, homily, instruction, lecture,
   lecture-demonstration, lesson, moral, moral lesson, morality,
   moralization, object lesson, objurgation, preach, preachification,
   preaching, preachment, prelection, rating, rebuke, recital,
   recitation, religious discourse, remonstrance, remonstration,
   reprehension, reprimand, reproach, reprobation, reproof, reproval,
   scolding, sermonette, sermonizing, set task, skull session,
   spanking, speech, talk, talking-to, task, teaching, tirade,
   travelogue, upbraiding