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From 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]