Brilliance is very great brightness — light strong enough to make you squint. Sun on fresh snow, a diamond turned under a lamp, headlights coming at you on a wet road.
The other half of the word is cleverness: the brilliance of an idea, of a performance, of a young mathematician who sees the answer before anyone has finished the question. Brilliant does the same double duty, which is why a brilliant light and a brilliant plan are both ordinary English. It is uncountable and it takes OF — the brilliance of the colours, her brilliance as a teacher — so there is no such thing as a brilliance. Jewellers use it as a technical word for how much light a cut stone throws back at the eye. When you want the neutral, measurable kind of light, say brightness; brilliance is praise.
1nounbrillianceVery great brightness.
“The brilliance of the snow made her eyes water.”
2noungeniusOutstanding cleverness or skill.
“There is real brilliance in the second half of the book.”
synonyms: genius
brilliance is a quality of brightness or of mind.
Brilliance is brilliant plus the ending -ance, which turns a quality into a thing you can name and count sentences around. French keeps the same word as brillance. The shape of the build tells you the order of events: things shone brilliantly first, and only then did people need a noun for the shining itself.
French
brillant
English
brilliant
English, today
brilliance
Built from
From brilliant + ance. Compare French brillance. brilliant Borrowed from French brillant. French brillant (late 17th century), present participle of the verb briller, from Italian brillare, possibly from Latin berillus, beryllus (“a beryl, gem, eyeglass”), from Ancient Greek βήρυλλος (“beryl”). by surface analysis, brill(er) + -i- + -ant.
Etymology from Wiktionary, CC BY-SA