Brilliant describes something super bright, like intense lights at a football stadium, a super sparkly diamond, or the student who graduates from Harvard at age 13.
Brilliant comes from the Italian word brillare, to "sparkle" like those bright lights, diamonds, and brainiac kid. British people love to say things are "Brilliant!" in the same way Americans say "Awesome!" Brilliant describes anything sparkly, fabulous, or full of light. A bright color is brilliant, too, like the brilliant orange of a setting sun.