Synesthesia - Wikipedia, the free encyclopedia: "Synesthesia (also spelled synæsthesia or synaesthesia, plural synesthesiae) -- from the Greek syn- meaning union and aesthesis meaning sensation -- is a neurological condition in which two or more bodily senses are coupled. In a form of synesthesia known as grapheme → color synesthesia, letters or numbers may be perceived as inherently colored, while in ordinal linguistic personification, numbers, days of the week and months of the year evoke personalities. While cross-sensory metaphors are sometimes described as 'synesthetic,' true neurological synesthesia is involuntary and occurs in slightly more than four percent of the population (1 in 23 persons) across its range of variants (Simner et al. in press). It runs strongly in families, possibly inherited as an X-linked dominant trait. Synesthesia is also sometimes reported by individuals under the influence of psychedelic drugs, after stroke or as a consequence of blindness or deafness. Synesthesia that arises from such non-genetic events is referred to as adventitious synesthesia to distinguish it from the more common congenital forms of synesthesia."