The fundamental note.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
The key or fundamental note of a piece of music.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
Fundamental note of a given scale or musical piece.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)
The basal note in a system of musical notes; ruling principle.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)

Usage examples:

Stupidity is really the keynote of Marguerite's character.
- Memoirs of an American Prima Donna by Clara Louise Kellogg
A poem written by him in 1773, entitled " The Dying Negro," has been described as supplying the keynote of the anti- slavery movement.
- The World's Greatest Books, Vol III by Arthur Mee and J.A. Hammerton, Eds.
" Ligeia," perhaps standing next, incorporating " The Conqueror Worm" as its keynote portrays the terrific struggle of a woman's will against death.
- Selections From Poe by J. Montgomery Gambrill