A "flipping" optical illusion
is an illusion
that can appear in two different ways, so the image "flips" in your mind. Examples are the Necker Cube
or the following ASCII
\ / \ /
Are the longer fins pointing inwards or outwards from the screen?
| / /
Are these stairs on the floor or in the ceiling?
If you stare at such a 2D image, your mind makes up a 3D representation of it. The problem is that the figure is ambigous; in 3 dimensions, it can be interpreted in two different ways. These two interpretations are the ones your mind flips between.
A similar phenomenon (but not related to 2D/3D) is the Pretty Woman/Old Hag imgage