Synesthesia

syn-es-the-sia n. Physiol. Sensation produced at a point other than
or remote from the point of stimulation, as of a sound from color (fr. Gk, syn = together + aisthesis = to perceive).

Synesthesia is an involuntary joining in which the real information of one sense is accompanied by a perception in another sense. In addition to being involuntary, this additional perception is regarded by the synesthete as real, often outside the body, instead of imagined in the mind's eye. It also has some other interesting features that clearly separate it from artistic fancy or purple prose. Its reality and vividness are what make synesthesia so interesting in its violation of conventional perception. Synesthesia is also fascinating because logically it should not be a product of the human brain, where the evolutionary trend has been for increasing separation of function anatomically.