IPA: /ˈbrɪljənsi/
KK: /ˈbrɪljəntsi/
The quality of being very bright or showing great intelligence and talent.
The artist's brilliancy was evident in every stroke of her brush.
Brilliancy is formed from "brilliant" (from Latin "brillare", meaning to shine) and the suffix "-ancy" (meaning the state or quality of). The word refers to the state of being brilliant or shining brightly.
Think of something that 'shines' ('brillare') and the '-ancy' indicating a state. This helps you remember that 'brilliancy' means the quality of shining brightly.