FIDE Rules of chess: 3.9
The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to the square occupied by the king because they would then leave or place their own king in check. No piece can be moved that will either expose the king of the same colour to check or leave that king in check.
Think of each move as a span of time long enough such that the king can escape if the opponent's king is captured. Sure, you want to capture my king, but I've already left the battlefield with my king safe and yours captured.
The king is said to be 'in check' if it is attacked by one or more of the opponent's pieces, even if such pieces are constrained from moving to the square occupied by the king because they would then leave or place their own king in check. No piece can be moved that will either expose the king of the same colour to check or leave that king in check.
Think of each move as a span of time long enough such that the king can escape if the opponent's king is captured. Sure, you want to capture my king, but I've already left the battlefield with my king safe and yours captured.