The trouble with Dolly's definition is that when there are three or four or five etc. notes in harmony loving each other, the concept is in dissonance with the moral values of the 56 some million Americans who voted for Karl Rove. (Sorry, Sarah, this is not scientific either but I could not let it just slip by...)
Pierre Divenyi
What a coincidence! In today's Family Circus (a newspaper comic; archives may be viewed at http://www.kingfeatures.com/features/comics/familyc/about.htm), Dolly is sitting at a piano and tells Jeffy, "Harmony is when you find two notes that love one another."
I'm sure someone out there has a more scientific definition, but I couldn't let this odd synchronicity go without comment. :)