Music . the note or tone on which a key or system of tones is founded; the tonic.

the main idea or central principle of a speech, program, thought, action, etc.

the policy line to be followed, as by a party in a political campaign, that is set forth authoritatively in advance by an address or other formal announcement.

keynote address.

to announce the policy of (a political party, campaign, assembly, etc.); deliver a keynote address at

The governor will keynote the convention.

to serve as the keynote for.

Music . to give the keynote of.

to provide a keynote, esp. a keynote address

He refused an invitation to keynote.

noun

basic idea , basis , center , core , cornerstone , criterion , crux , essence , gist , heart , idea , kernel , keystone , linchpin , mainspring , marrow , measure , motive , nub , pith , principle , root , source , spring , standard , substance , feature , theme , tone , topic