The Collaborative International Dictionary of English v.0.48:

Topic \Top"ic\, n. [F. topiques, pl., L. topica the title of a
   work of Aristotle, Gr. topika`, fr. topiko`s of or for place,
   concerning to`poi, or commonplaces, fr. to`pos a place.]
   (a) One of the various general forms of argument employed in
       probable as distinguished from demonstrative reasoning,
       -- denominated by Aristotle to`poi (literally, places),
       as being the places or sources from which arguments may
       be derived, or to which they may be referred; also, a
       prepared form of argument, applicable to a great variety
       of cases, with a supply of which the ancient rhetoricians
       and orators provided themselves; a commonplace of
       argument or oratory.
   (b) pl. A treatise on forms of argument; a system or scheme
       of forms or commonplaces of argument or oratory; as, the
       Topics of Aristotle.
       [1913 Webster]

             These topics, or loci, were no other than general
             ideas applicable to a great many different
             subjects, which the orator was directed to consult.
                                                  --Blair.
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             In this question by [reason] I do not mean a
             distinct topic, but a transcendent that runs
             through all topics.                  --Jer. Taylor.
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   2. An argument or reason. [Obs.]
      [1913 Webster]

            Contumacious persons, who are not to be fixed by any
            principles, whom no topics can work upon. --Bp.
                                                  Wilkins.
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   3. The subject of any distinct portion of a discourse, or
      argument, or literary composition; also, the general or
      main subject of the whole; a matter treated of; a subject,
      as of conversation or of thought; a matter; a point; a
      head.
      [1913 Webster]

   4. (Med.) An external local application or remedy, as a
      plaster, a blister, etc. [Obsoles.] --Wiseman.
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The Collaborative International Dictionary of English v.0.48:

Topic \Top"ic\, a.
   Topical. --Drayton. Holland.
   [1913 Webster]

WordNet (r) 3.0 (2006):

topic
    n 1: the subject matter of a conversation or discussion; "he
         didn't want to discuss that subject"; "it was a very
         sensitive topic"; "his letters were always on the theme of
         love" [syn: subject, topic, theme]
    2: some situation or event that is thought about; "he kept
       drifting off the topic"; "he had been thinking about the
       subject for several years"; "it is a matter for the police"
       [syn: topic, subject, issue, matter]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "topic":
   action, anagnorisis, angle, architectonics, architecture, argument,
   atmosphere, background, bone of contention, catastrophe, catechism,
   characterization, color, complication, continuity, contrivance,
   cross-interrogatory, cross-question, debating point, demand,
   denouement, design, development, device, episode, fable,
   falling action, feeler, gimmick, head, incident, inquiry,
   interrogation, interrogative, interrogatory, issue, keynote,
   leader, leading question, line, local color, matter, mood,
   moot point, motif, motive, movement, mythos, peripeteia, plan,
   plot, point, point at issue, point in question, problem,
   proposition, query, question, question at issue, question mark,
   quodlibet, recognition, rising action, scheme, secondary plot,
   slant, story, structure, subject, subject matter, subplot, switch,
   text, thematic development, theme, thesis, tone, trial balloon,
   twist, vexed question