Definition of "brilliancy"

[…] to assist greatly in the consumption of the lamp, and to diminish the aqueous precipitations produced on the interior of the lantern glass, which has the effect of reducing more or less the brilliancy of the light.

1871, Léonce Reynaud, Memoir Upon the Light-house Illumination of the Coasts of France, page 110

[H]is ideal of grandeur was a splendid façade, diffusing its brilliancy outward too, irradiating hospitality.

1876–1877, Henry James, Jr., chapter III, in The American, Boston, Mass.: James R[ipley] Osgood and Company, […], published 5 May 1877, pages 56–57

Surely it must have been his brilliancy that accounted for Vardon standing out as a stroke or two better than such a model of steadiness as J. H. Taylor[.]

1922, W. K., “Has the Standard of Professional Golf Improved?”, in Baily's Magazine of Sports and Pastimes‎, page 125

Bobby Fischer, one of the greatest chess players who ever lived, died at the age of sixty-four from kidney failure. He had lived one year for each square on a chessboard. First of all, you should learn the term “brilliancy.” It refers to a game that is played with spectacular strategy and beauty, typically featuring unexpected moves and ingenious ideas. In other words, the game is extraordinary! At age thirteen, Bobby Fischer won a brilliancy against twenty-six-year old Donald Byrne.

2015, Michael Hearst, Extraordinary People: A Semi-Comprehensive Guide to Some of the World's Most Fascinating Individuals, Chronicle Books, page 38