The term 'Illusory Sight on Mars' refers to the belief that there were canals on the surface of Mars. This belief was popularized in the late 19th century by Italian astronomer Giovanni Schiaparelli, who observed what he believed to be linear features on the surface of Mars. These features were later determined to be an optical illusion, and the term 'Illusory Sight on Mars' was coined to refer to this phenomenon. The crossword solution for this term is CANAL, which is a man-made waterway used for transportation, irrigation, or drainage.