Marcus Gunn pupil is a term used to describe a condition where the pupil of one eye appears to constrict less in response to light compared to the opposite eye. This condition is caused by a neurological issue and is highly uncommon. However, there are several synonyms or alternative terms that experts may use to describe this condition. These include afferent pupillary defect, paradoxical pupillary reaction, relative afferent pupillary defect, or simply, a Marcus Gunn phenomenon. These terms are used interchangeably to describe this rare medical condition and help medical professionals better understand this rare pupil irregularity.