keynote

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The tonic of a musical key.
• noun A prime underlying element or theme.
• transitive verb To give or set the keynote of.
• transitive verb Informal To give a keynote address at.

from The Century Dictionary.

• noun In music, the tone on which a key (tonality) is founded; a tonic. See key, 7 .
• noun A central principle or idea; the pivotal point in a system, a composition, or a course of action; a controlling thought.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.
• noun The fundamental fact or idea; that which gives the key.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun music The note on which a musical key is based.
• noun The main theme of a speech, a written work, or a conference.
• noun A speech that sets the main theme of a conference or other gathering; a keynote speech or keynote address.
• verb transitive To deliver a speech that sets the main theme of a conference or other gathering.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb give the keynote address to (an audience)
• noun (music) the first note of a diatonic scale
• noun a fundamental or central idea
• verb set the keynote of
• noun the principal theme in a speech or literary work