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)