Chess Glossary
Terms you'll run into around the site and at the board. For a full walkthrough of the rules, see Rules or Learn.
Check
A direct attack on the king. You must get out of check immediately — by moving the king, blocking the attack, or capturing the attacking piece.
Checkmate
A check with no legal way out. The game ends immediately, and the side that delivered it wins.
Stalemate
The side to move has no legal moves but isn't in check. The game is a draw — no one wins.
Castling
A special one-time move where the king moves two squares toward a rook and that rook jumps to the square beside it, usually to tuck the king away safely. Only legal if neither piece has moved, the squares between them are empty, and the king isn't in, through, or into check.
En passant
A special pawn capture: if an opponent's pawn moves two squares from its start and lands beside your pawn, you may capture it as if it had only moved one square — but only on the very next move.
Fork
A single move that attacks two or more enemy pieces at once, so the opponent can't save them both.
Pin
A piece can't (or shouldn't) move because doing so would expose a more valuable piece behind it to attack — most severely when that piece is the king, which makes the move illegal.
Skewer
Like a pin in reverse: an attack that forces a valuable piece to move out of the way, exposing a less valuable piece behind it to capture.
Zugzwang
A position where any legal move makes things worse — the player would rather pass, but chess doesn't allow that.
Opening, middlegame, endgame
The three broad phases of a game: the opening (development and king safety), the middlegame (plans, tactics, attacks), and the endgame (usually fewer pieces, where king activity and pawns matter most).
Tempo
A "move" of relative time. Gaining tempo means making progress while also forcing a response, effectively getting a free move; losing tempo means wasting one.
FEN (Forsyth-Edwards Notation)
A compact text format that describes a full board position — piece placement, whose turn it is, castling rights, and more — in a single line.
PGN (Portable Game Notation)
The standard text format for recording a full game's moves, used across the site's Games Database, Collections, and Review — and the format most other chess sites export to as well.
Algebraic notation
The standard way to write chess moves, like Nf3 or e4 — a letter for the piece (pawns have none) followed by the square it moves to.
Rating (Elo)
A number that estimates playing strength based on results against other rated players. Win against a higher-rated opponent and your rating rises more; the same loss costs a higher-rated player more than it costs a lower-rated one.
Time control
How much time each player gets for the whole game (and sometimes per move), e.g. bullet (under 3 minutes), blitz (3–10 minutes), rapid (10–30 minutes), or classical (30+ minutes).
Book move / theory
A move that's part of well-studied opening theory — "in the book" — as opposed to a novelty (a new idea played outside established theory).
Blunder
A serious mistake that significantly worsens a player's position — typically the biggest kind of error a game-review evaluation bar will flag.