An invisible wall is a tile that acts exactly like a wall, but cannot be seen. Unlike the hidden wall, it does not reveal itself when touched by Chip, or by any object.
The above symbol depicting a black X inside a box is used in level editors; in actual play, the invisible wall looks like a normal floor space. Only monster rebounds, "oof" sounds from Chip, and failure to move a block onto the space will divulge its true nature.
Note that if an invisible wall is revealed on the lower layer of the grid by Chip under a removable tile (excluding boots) or floor, it will not take effect on this turn.