A distorted perception of reality; unlike delusions, illusions are generally shared by most people, as in a 3D “hidden picture” puzzle.
Example: A breeze stirs the curtains, you see the motion out of the corner of your eye, and you briefly think you see a lurking attacker.
Grinnell, R. (2011). Illusion. Psych Central. Retrieved on May 25, 2015, from http://psychcentral.com/encyclopedia/2008/illusion/