An
optical illusion showing two
parallel lines on a background of lines protruding from a single point between the two parallel ones. The two parallel lines seem to be bulging out from the center.
This illusion is in the same class as the Poggendorff Illusion, Muller-Lyer Illusion and the Zollner Illusion as they are all illusions where the perception of an object is altered by its background.
Unfortunately, I haven't been able to recreate this illusion in ASCII. To give you an idea of what I'm talking of, though; it looks almost but not entirely, completely unlike this:
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*
/|\
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/ | \
See a much better example at http://www.uic.edu/depts/mcop/patients/illusions/parallel.htm