The Collaborative International Dictionary of English v.0.48:

Classic \Clas"sic\ (kl[a^]s"s[i^]k), Classical \Clas"sic*al\, a.
   [L. classicus relating to the classes of the Roman people,
   and especially to the frist class; hence, of the first rank,
   superior, from classis class: cf. F. classique. See Class,
   n.]
   1. Of or relating to the first class or rank, especially in
      literature or art.
      [1913 Webster]

            Give, as thy last memorial to the age,
            One classic drama, and reform the stage. --Byron.
      [1913 Webster]

            Mr. Greaves may justly be reckoned a classical
            author on this subject [Roman weights and coins].
                                                  --Arbuthnot.
      [1913 Webster]

   2. Of or pertaining to the ancient Greeks and Romans, esp. to
      Greek or Roman authors of the highest rank, or of the
      period when their best literature was produced; of or
      pertaining to places inhabited by the ancient Greeks and
      Romans, or rendered famous by their deeds.
      [1913 Webster]

            Though throned midst Latium's classic plains. --Mrs.
                                                  Hemans.
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            The epithet classical, as applied to ancient
            authors, is determined less by the purity of their
            style than by the period at which they wrote.
                                                  --Brande & C.
      [1913 Webster]

            He [Atterbury] directed the classical studies of the
            undergraduates of his college.        --Macaulay.
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   3. Conforming to the best authority in literature and art;
      chaste; pure; refined; as, a classical style.
      [1913 Webster]

            Classical, provincial, and national synods.
                                                  --Macaulay.
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   Classicals orders. (Arch.) See under Order.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Classic \Clas"sic\, n.
   1. A work of acknowledged excellence and authority, or its
      author; -- originally used of Greek and Latin works or
      authors, but now applied to authors and works of a like
      character in any language.
      [1913 Webster]

            In is once raised him to the rank of a legitimate
            English classic.                      --Macaulay.
      [1913 Webster]

   2. One learned in the literature of Greece and Rome, or a
      student of classical literature.
      [1913 Webster]

WordNet (r) 3.0 (2006):

classic
    adj 1: of recognized authority or excellence; "the definitive
           work on Greece"; "classical methods of navigation" [syn:
           authoritative, classical, classic, definitive]
    2: of or relating to the most highly developed stage of an
       earlier civilisation and its culture; "classic Cinese
       pottery" [syn: classical, classic] [ant: nonclassical]
    3: of or pertaining to or characteristic of the ancient Greek
       and Roman cultures; "classical mythology"; "classical [syn:
       classical, classic, Greco-Roman, Graeco-Roman,
       Hellenic]
    n 1: a creation of the highest excellence
    2: an artist who has created classic works

Moby Thesaurus II by Grady Ward, 1.0:

151 Moby Thesaurus words for "classic":
   French literature, Renaissance literature, ageless,
   ancient literature, archetypal, archetype, art object,
   belles lettres, best seller, book, bound book, brainchild,
   bric-a-brac, capital, champion, classical, classical music,
   classics, coloring book, composition, concert music, concertino,
   concerto, concerto grosso, contemporary literature, creation,
   deathless, definitive, definitive work, design, enduring, epitome,
   erotic literature, erotica, excellent, exemplar, exemplary, famous,
   fine, first-rate, folio, folk literature, great work, grotesque,
   hardback, humane letters, ideal, immortal, juvenile, juvenile book,
   kitsch, leading, legendary, letters, limp-cover book, literature,
   longhair music, magnum opus, master, masterpiece, masterwork,
   medieval literature, mirror, mobile, model, museum piece,
   national literature, nonbook, notable, notebook, noteworthy, novel,
   nude, obscene literature, old master, opus, opuscule, opusculum,
   outstanding, paperback, paradigm, paradigmatic, paragon, pasticcio,
   pastiche, pattern of perfection, picture book, piece,
   piece of virtu, playbook, pocket book, polite literature,
   pop literature, popular literature, pornographic literature,
   pornography, prayer book, prime, production, prototypal, prototype,
   prototypical, psalmbook, psalter, pseudonymous literature,
   publication, quintessence, quintessential, representative,
   republic of letters, rhapsody, scatological literature,
   semiclassical music, serial, serious literature, sinfonietta,
   sketchbook, soft-cover, songbook, stabile, standard, standard work,
   statue, still life, storybook, study, superior, symphonia,
   symphonic music, symphonic ode, symphony, time-honored, timeless,
   title, tome, tone poem, top, top-notch, tour de force, trade book,
   travel literature, underground literature, undying, venerable,
   very model, virtu, volume, wisdom literature, work, work of art,
   writing



The Free On-line Dictionary of Computing (30 December 2018):

classic

    An adjective used before or after a noun to describe
   the original version of something, especially if the original
   is considered to be better.

   Examples include "Star Trek Classic" - the original TV series
   as opposed to the films, ST The Next Generation or any of the
   other spin-offs and follow-ups; or "PC Classic" - IBM's
   ISA-bus computers as opposed to the PS/2 series.

   (1996-10-27)