They all measure the same thing
Inaccuracy, mistake and blunder are three points on one scale: how much evaluation your move threw away compared to the engine's best move. The bigger the drop, the worse the label. That drop is often measured in centipawns (hundredths of a pawn) — see centipawn loss explained.
The three labels
- Inaccuracy (?!): a small slip. You kept most of your position but chose a slightly worse move — a modest evaluation loss.
- Mistake (?): a real error. You gave up a meaningful chunk of your advantage, or handed your opponent a clear edge.
- Blunder (??): a serious error that can swing or lose the game — hanging a piece, missing a mate, or walking into a decisive tactic.
Why the same move can change label
The label depends on the position, not the move in isolation. Dropping from +6 to +4 is still winning, so a review may barely flag it. Dropping from +1 to −3 flips the game and is a blunder. This is why a move that looks the same can be judged very differently: what matters is how much the evaluation actually changed and what it changed to.
What to do with them
Blunders are where your rating is won and lost, so start there. Run a free game review, jump to each blunder, and understand the move you missed. Then train the exact positions as blunder puzzles from your own games so the pattern sticks.