What is a best move in chess?
A best move is one that loses almost nothing compared with the engine's preferred line — under 10 centipawns. It is usually the engine's first choice, but any move that close to it counts.
Threshold
Best — under 10 centipawns lost against the engine’s best line.
How it is measured
Every move is scored by what it costs. The engine evaluates the position before the move and after it, and the difference — from the moving side's point of view — is the centipawn loss. A centipawn is a hundredth of a pawn, so 100 centipawns is a whole pawn's worth of advantage.
A best move scores under 10. That is deliberately not zero: engines disagree with themselves at different depths, and calling only the single top move 'best' would grade the same position differently depending on how long the search ran.
Best is not the same as only
Many positions have several moves within a few centipawns of each other. All of them grade as best, which is the honest result — a position with three equally good continuations does not have one right answer.
This matters most in quiet positions. Sharp tactical ones often do have a single move that holds, and there the band is narrow because everything else genuinely loses something.
Frequently asked questions
Is the best move always the engine's first choice?
Usually, but not necessarily. Anything within 10 centipawns of the top line grades as best, so a position with several near-equal continuations will have several best moves.
Does playing all best moves mean 100% accuracy?
In this app's scoring, yes — best and good moves carry no accuracy penalty. Other sites weight their accuracy differently, which is why the same game can score differently on each.