An invisible wall is a tile that acts exactly like a wall, but cannot be seen. As unlike the hidden wall, it does not reveal itself when touched by Chip, or by any object.
In actual play, the invisible wall looks like a normal floor space, although monster rebounds, "oof" sounds from Chip, and failure to move a block onto the space will divulge its true nature. The black X inside a box is the symbol for it in maps and level editors.
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.