You mean something like this? Where it's white's move and the knight can't be moved?
[FEN "4k3/8/4r3/8/4N3/8/8/4K3 w - - 0 1"]
That is called to "pin" the piece (the knight in this case) and specifically an "absolute pin" because the king is protected and it's illegal to move the piece. So you'd say that the knight is "absolutely pinned".