A brilliancy is a bug
A brilliancy is an original and imaginative move in chess. When a move is brilliant it is marked as (!!). A register of surprise.
After I finish a match, my chess application tells me how many brilliancies I played, how many best moves, how many inaccuracies, how many mistakes, and how many blunders.
The best move is the move that puts the player in best position of winning, according to the engine. A brilliancy is a move that the engine didn’t anticipate. The engine only recognizes a brilliancy after-the-fact.
A brilliancy is better than the best move.
As chess engines get better brilliancies become rarer. I’ve never played a brilliancy.
Today brilliancies mostly occur by accident. Chess engines treat them like bug reports; when one occurs, the engine corrects itself to ensure that it doesn’t happen again.