Anamorphosis - Oxford Definiton

[From the Greek anamorphsis, ‘transformation’] a visual trick consisting of a deliberate distortion (such as elongation) of an object represented in a painting or drawing which, if viewed from a certain point or reflected in a curved mirror, appears normal again. Famous examples include the skull in Holbein's painting of The Ambassadors of 1533 (National Gallery, London) or the portrait of Edward VI (National Portrait Gallery, London).