A meaningful piece sacrifice that creates a decisive tactical advantage, turns a losing game, or leads to checkmate, surprising both opponent and analysis engine, and is generally more than just the best move.
Brilliant moves are definitely the rarest move type because of the requirements a certain move needs to meet to be a brilliant, the requirements are (potentially not completely correct) for the move to be surprising to the engine analyzing the game, the only or one of the only moves that either keeps advantage, keeps even evaluation, gains high/any advantage, or gains enough evaluation to make it even. It is also generally a sacrifice that leads to execution of a long term plan that has a greater positive effect than the sacrifices negative effect. They are marked as surprising because they involve short term loss and people don't generally calculate those lines to see what can happen next because in lines with short term loss, there is usually just more loss or at least no gain.
Sorry for the long post, just wanted to be thorough with my information on brilliant moves.
TL;DR
Brilliant are generally sacrifices that are of the only good moves or the only good move in a position that the engine finds surprising.
Brilliant move for me is bank rank mate, wonderful!!!
How is that brilliant, it must’ve been REALLY hard to see otherwise it couldn’t have been brilliant (Im assuming your talking about the move to get the mate, no other moves)?