Synesthesia is a stable neurological trait in which stimulation of one sensory or cognitive pathway involuntarily triggers experience in another. Common forms include grapheme-color synesthesia (letters and numbers have colors), chromesthesia (sounds evoke colors), and number-form synesthesia (numbers map to spatial arrangements). Estimates of prevalence vary, but a frequently cited figure is around 2–4% of the population for at least one form.
Synesthesia is distinct from hyperphantasia: hyperphantasia is vivid voluntary imagery, while synesthetic experiences are automatic cross-sensory pairings triggered by an external or imagined stimulus. The two can co-occur but represent different cognitive phenomena. Some research suggests synesthetes have stronger-than-average mental imagery on certain measures, but synesthesia itself is not measured by Imagination Index. The full picture of an individual's imagination requires looking at vividness (Imagination Index), cross-sensory pairings (synesthesia tests), and other related traits like inner experience type.