Is it really check if a King is attacked, but the attacking piece can't move?

Question: Is it really check if a King is attacked, but the attacking piece can't move?

Answer:

Yes. If a King is attacked, it is in check. It makes no difference that the attacking piece can't move because it blocks an attack on its own King.

Similarly, if a piece is blocking an attack on its own King, the other King can't move into an attack by that piece. A player can never leave the King under attack by an opponent's piece. It doesn't matter what that piece is doing.

To confirm this yourself, try setting up different positions on your chess computer. The computer won't let you move into check under any circumstances. It also won't let you set up a position where a King is in check, but it is the other player's turn to move.