Source : Webster's Revised Unabridged Dictionary (1913)

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}.

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

Source : WordNet®

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)