The Collaborative International Dictionary of English v.0.48:

Keynote \Key"note`\, n.
   1. (Mus.) The tonic or first tone of the scale in which a
      piece or passage is written; the fundamental tone of the
      chord, to which all the modulations of the piece are
      referred; -- called also key tone.
      [1913 Webster]

   2. The fundamental fact or idea; that which gives the key;
      as, the keynote of a policy or a sermon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

keynote
    n 1: the principal theme in a speech or literary work
    2: a fundamental or central idea
    3: (music) the first note of a diatonic scale [syn: tonic,
       keynote]
    v 1: set the keynote of; "Comfort keynotes this designer's Fall
         collection"
    2: give the keynote address to (an audience)

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "keynote":
   aroma, attribute, badge, banner, be characteristic, brand, cachet,
   cast, character, characteristic, characterize, configuration, cut,
   define, demarcate, describe, device, differentia, differential,
   differentiate, distinctive feature, distinguish, dominant, earmark,
   feature, figure, flavor, gust, hallmark, idiocrasy, idiosyncrasy,
   image, impress, impression, index, indicant, indicator,
   individualism, insignia, key, key signature, lineaments, major,
   major key, mannerism, mark, mark off, mark out, marking, measure,
   mediant, minor, mold, nature, note, odor, particularity,
   peculiarity, pedal point, picture, property, quality, quirk,
   representation, representative, savor, seal, set apart,
   set the pace, set the tone, shape, sigil, sign, signal, signature,
   singularity, smack, sound the keynote, specialty, stamp,
   subdominant, submediant, subtonic, supertonic, sure sign, symptom,
   taint, tang, taste, telltale sign, token, tonality, tonic,
   tonic key, trait, trick