Analysis tools grade bad moves in three tiers by how much evaluation they lose: an inaccuracy (?!) is a small slip, a mistake (?) is a clearly inferior move, and a blunder (??) is a game-changing error. The boundaries are centipawn thresholds and differ slightly between platforms.
Run any game through analysis and you'll get a report card: two inaccuracies, one mistake, one blunder. The labels look official, but they are just bands on a single scale — how much evaluation a move threw away compared to the engine's best move.
Inaccuracy (?!) — a move that loses a small amount of evaluation, commonly around half a pawn. The position is still fine; you simply had something better. Strong players produce inaccuracies in every game.
Mistake (?) — a clearly inferior move, typically costing around one to two pawns of evaluation. Mistakes hand the opponent a real advantage: a lost pawn, a dominant piece, a broken king position.
Blunder (??) — a move that changes the likely outcome of the game. Hanging material, allowing a decisive tactic, missing a win. See the full blunder entry for why these decide most amateur games.
The exact cutoffs are measured in centipawns and vary by platform — modern tools also weigh how much the move changed the expected result, not just the raw evaluation. That is why the same game can show different counts on Chess.com, Lichess, and other analyzers. Don't compare numbers across sites; compare your games against your own baseline.
Not equally. A common trap is polishing inaccuracies while still hanging pieces every third game.
For most players below roughly 1800, the order of business is clear: eliminate blunders first, reduce mistakes second, and ignore inaccuracies almost entirely. One blunder outweighs ten inaccuracies, and blunders repeat in patterns that you can actually train against.
A good habit when reviewing your games: skip straight to the blunders and mistakes, and for each one ask what you were thinking at that moment — not just what the engine preferred.
Chessdock automates exactly this: it pulls your games from Chess.com or Lichess, finds the moves that mattered, and turns your real mistakes into puzzles you can retrain until the pattern sticks.