The quality or state of being brilliant; brightness or outstanding excellence.
From 'brilliant' plus the abstract noun suffix '-ness.' This forms a parallel quality noun to 'brilliance' and 'brilliancy.'
English gives us three different words for the same concept—brilliance, brilliancy, and brilliantness—showing how 19th-century writers loved finding new ways to describe excellence and brightness.
Complete word intelligence in one call. Free tier — 50 lookups/day.