An invisible wall is a tile that acts exactly like a wall, but cannot be seen. Unlike the hidden wall, it does not permanently reveal itself when touched by Chip, or by any object. In Chip's Challenge 2, invisible walls will appear as a wall for a brief moment after being touched, then revert to its floor appearance.
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. in Chip's Challenge 2, identifying it is made slightly easier, as it will briefly appear as a visible wall when pressed against by Chip, Melinda, either of their mirror counterparts, or a rover. Collecting the secret eye will cause it to appear as it does in the editor.