Fork
A single piece attacks two or more enemy units at once, forcing the opponent to lose material on the response.
A fork is the most direct tactical idea in chess. A single piece simultaneously attacks two or more enemy pieces, and the opponent cannot defend both. Whichever piece is moved to safety, the other one is captured.
The knight is the most notorious forking piece because it attacks squares that no other piece can reach, and a knight on the right square can hit a queen and a king at the same time. A knight fork that hits the enemy king and queen is called a royal fork and is almost always decisive.
Pawns fork too. After e4-e5 attacking both a knight on f6 and a bishop on d6, Black usually loses a piece. The pawn fork is harder to foresee than the knight fork because the pawn moves so quietly, and many games have been decided by a single under-the-radar pawn push.
Every piece can fork, but the fork’s power comes from a piece’s mobility relative to the defenders. A queen fork is rarely decisive because the queen has many targets but is itself vulnerable; a rook or bishop fork along an open line is often decisive because the opponent cannot interpose.
The defence against a fork is anticipation. Once a fork is on the board, it is usually too late.