Real Dictionary

keynote
The keynote of a policy, speech, or idea is the main theme of it or the part of it that is emphasized the most.

keynote in American English
the lowest, basic note or tone of a musical scale, or key; tonic
to give the keynote of
noun: the note or tone on which a key or system of tones is founded; the tonic

keynote in British English
to deliver a keynote address to (a political convention, etc)
noun: a central or determining principle in a speech, literary work, etc