An ambiguous image is an image that has more than one interpretation. A classic example is Rubin's vase which can be seen as a vase or two faces. Human vision typically flips back and forth, largely uncontrollably, between the interpretations. In real situations the ambiguity either resolves naturally or one moves one's head to get a different view. The latter is emulated in active visions where the camera moves.
Used in Chap. 12: page 161; Chap. 15: page 232; Chap. 22: page 348
Also known as in computer vision
Rubin's vase. Source: NevitNevit Dilmen, CC BY-SA 3.0 via Wikimedia Commons