The term synesthesia (Greek: syn = at the same time, aesthesia = perception) indicates the simultaneous, involuntary perception of different, unrelated sensory impressions and is therefore also referred to as a blending of the senses. Thus, language, for example, can arouse the perception of color, or odors may generate geometric figures.

If one regards synesthesia as a topic of research, two discrete approaches need to be distinguished. In a neurological sense synesthesia differs fundamentally from efforts to expose similarities between sense modalities, in particular between sounds and colors in terms of a hidden correspondence or a higher formula (Johann Wolfgang von Goethe) as well as from the stylistic device primarily found in romantic and symbolist literature.

Humankind has occupied itself with models of correspondence and color-sound analogies since Aristotle, while in the neurological sense synesthesia did not become an object of scientific interest until the mid to late 19th century. The following entry deals with the history of synesthesia from a neuroscientific perspective.