noun

• (neurology, psychology) A neurological or psychological phenomenon whereby a particular sensory stimulus triggers a second kind of sensation.
  "M. [Théodore] Flournoy includes all the phenomena of "Colored Hearing" and of "Mental Forms" under the convenient and adequate name Synæsthesia—in place of which, to be sure, he himself usually employs the less defensible term Synopsie. […] The phenomena of synæsthesia are divided into three main groups: "photisms," among which are included, as by [Eugen] Bleuler and Lehmann, all the varieties of pseudo-chromesthesia; "Schemes," comprising not only "forms" (diagrammes) associated with series of words or numbers, but "symbols," or particular figures associated with single letters, numerals, colors and the like; and "personifications," in which the associated factor is no mere color or form, but has become richer and more concrete."
• (by extension) The association of one sensory perception with, or description of it in terms of, a different perception that is not experienced at the same time.
  "On a phonemic level, phenomena of synesthesia have often been described and studied. Practically all children and a good many adults—though for the most part adults will deny it—spontaneously associate sounds, whether phonemes or the timbre of musical instruments, with colors and forms."
• (art, literature) A literary or artistic device whereby one kind of sensation is described in the terms of another.
  "Linking moods with colours is one example of synaesthesia. […] As a tool for improving writing, the idea of synaesthesia is versatile and innovative. Why use only visual references to describe visual impressions? Why just describe sounds using sound-related words? Play with cross-matching the senses."