Synaesthesia: A rhetorical trope involving shifts in imagery. It involves taking one type of sensory input (sight, sound, smell, touch, taste) and comingling it with another separate sense in an impossible way. In the resulting figure of speech, we end up talking about how a color sounds, or how a smell looks. When we say a musician hits a 'blue note' while playing a sad song, we engage in synaesthesia. When we talk about a certain shade of color as a 'cool green,' we mix tactile or thermal imagery with visual imagery the same way. When we talk about a 'heavy silence,' we also use synaesthesia. Examples abound: The scent of the rose rang like a bell through the garden. 'I caressed the darkness with cool fingers.' French poets, especially Baudelaire in Les fleurs du mal, have proven especially eager to use synaesthesia. The term itself is a fairly late addition to rhetoric and literary terminology, first coined in 1892, though examples of this figure of speech can be found in Homer, Aeschylus, Donne, Shelley, Crashaw, and scores of other writers and poets. See examples under tropes. (also spelled synesthesia, from Grk. 'perceiving together')
Synaesthesia: stimulus in one sensory field leads to a hallucination in another sensory field for example, a sound produces the sensation of a particular colour.
Synesthesia: A blending of different senses in describing something.