This word is moderately common in the psychological and artistic fields. It refers to a person who has some kind of cross-wiring in the brain, so that things which ought to be perceived by one sense are instead felt in another.
The most common form is for language, sounds and tastes to be sensed as colours. For a synaesthete, the words on this page might have colour associations; many musicians say that different keys evoke characteristic colours; or those affected may experience the sense of taste as hues. For any one individual, the transfer between senses is always the same, but there seems to be no accord between affected individuals about what sensation evokes what effect.
The science writer Alison Motluk, a synaesthete, has remarked that “The astonishing realisation is not that these characters are imbued with colours but rather a world could exist in which they were colour-free, neutral, characterless. It would be like finding out one day that, while you have been savouring the smells of freshly baked bread, of brandy, of chocolate, all your life, your friends have only been able to taste them”.
The term synaesthesia was coined at the end of the nineteenth century by Sir Francis Galton for what had been and sometimes is still called coloured hearing. He derived it on the model of anaesthesia from the Greek prefix syn–, for things that are like one another, plus aisthesis, meaning sensation.