Illusion/ɪˈluːʒn/ Definition 1 of 4Pro Tip 1/2Not Real Remember 'illusion' suggests something seems real but isn't, often due to circumstances. The shadows on the wall created the illusion of movement. Pro Tip 2/2Visual Deception An 'illusion' can be a visual trick that confuses the eyes and brain. The lines seem curved, but it's just an optical illusion.