Dr. Sean A. Day’s definition of synaesthesia from The Synesthesia List:
Synesthesia is the general name for a set (a “complex”) of over 80 related cognitive traits. Synesthesia may be divided into two general, somewhat overlapping groups. The first, sometimes called “synesthesia proper”, is when stimuli to a sensory input will also trigger sensations in one or more other sensory modes. The second group of synesthesia, called “cognitive” or “category synesthesia”, involves synesthetic additions to culture-bound cognitive categorizational systems. In simpler words, with this kind of synesthesia, certain sets of things which our individual cultures teach us to put together and categorize in some specific way, such as letters, numbers, or people’s names, also get some kind of sensory addition, such as a smell, color or flavor. The most common forms of cognitive synesthesia involve such things as colored written letter characters (graphemes), numbers, time units, and musical notes or keys. For example, the synesthete might see, about a foot or two before her, different colors for different spoken vowel and consonant sounds, or perceive numbers and letters, whether conceptualized or before her in print, as colored.
Synesthesia is additive; that is, it adds to the initial (primary) sensory perception, rather than replacing one perceptual mode for another. With my colored musical timbres, I both hear and “see” the sounds; the visual images don’t replace the audial sensations. Both sensory perceptions may thus become affected and altered in the ways they function and integrate with other senses. Synesthesia is generally “one-way”; that is, for example, for a given synesthete, tastes may produce synesthetic sounds, but sounds will not produce synesthetic tastes. However, there have been a few rare cases of synesthetes who have had “bi-directional” synesthesia, in which, for example, music induces (synesthetic) colors and seeing colors induces (synesthetic) sounds; the correspondences, however, may not be the same in both directions! Regarding synesthesia “proper”, stimuli to one sense, such as smell, are involuntarily simultaneously perceived as if by one or more other senses, such as sight or/and hearing.
The word “synesthesia” comes directly from the Greek (syn-) “union”, and (asthesis) “sensation”, thus meaning something akin to “a union of the senses”. “Synaesthesia” is the British English spelling of the word; in American English, it is often spelled “synesthesia”, without the “a”. The concept appears in other European languages, too: In Danish it is synae¦stesie . The Dutch word is synesthesie . In Finnish, synestesia. In French, it’s synesthésie , one type of which is audition colorée, “colored hearing”. In German, it’s Synästhesie, and colored hearing is Farbenhören. In Italian, sinestetici; in Polish, synestezja; in Russian, Синестезия (sinestezia); in Spanish, sinestesia; in Swedish, it’s synestesi.
Synesthesia has definite neurological components and is apparently partially heritable. The percentage of the general human population which has synesthesia varies with the type involved; estimates run from 4 in 100 for basic types of cognitive synesthesia (colored letters or musical pitches), to 1 in 3,000 for more common forms of synesthesia proper (colored musical sounds or colored taste sensations), to 1 in 15,000 or more for people with rare (such as one synesthete I know of who synesthetically tastes things she touches) or multiple forms of synesthesia proper. Perhaps more than half of all humans have a basic form of synesthesia in which they consider “higher” sounds to be “brighter” and “lower” sounds to be “darker”.