Definition from Wiktionary, the free dictionary
IPA (US): /ˈkiːˌnoʊt/
keynote (plural keynotes)
- (music) The note on which a musical key is based.
- The main theme of a speech, a written work, or a conference.
- A speech that sets the main theme of a conference or other gathering; a keynote speech or keynote address.
Derived terms 
speech that sets the main theme