The Delboeuf illusion is an optical illusion

Optical illusion

An optical illusion is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source...

 of relative size perception. In the best-known version of the illusion, two circles of identical size have been placed near to each other and one is surrounded by an annulus

Annulus (mathematics)

In mathematics, an annulus is a ring-shaped geometric figure, or more generally, a term used to name a ring-shaped object. Or, it is the area between two concentric circles...

; the surrounded circle then appears larger than the non-surrounded circle if the annulus is close, while appearing smaller than the non-surrounded circle if the annulus is distant. Recent work suggests it is caused by the same visual processes that cause the Ebbinghaus illusion

Ebbinghaus illusion

The Ebbinghaus illusion or Titchener circles is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size are placed near to each other and one is surrounded by large circles while the other is surrounded by small circles; the first...