From the Ancient Greek verb syn-aisthainomai, "to perceive, understand together, feel together, share the feeling", stems the noun syn-aìsthesis, "simultaneous feeling, perception".

Synesthesia, long regarded as a sort of condition (known as "Kandinskij's disease") is nowadays recognized as a widely experienced phenomenon. Much interest is lately being devoted to synesthesia, thanks to the recent neuroscience research that underscores its value in relation to brain plasticity, the concept that revolutionized all former knowledge on human brain.

Amongst the many sites dealing with synesthesia, see first Wikipedia's definition at:
http://en.wikipedia.org/wiki/Synesthesia. 

For neuroscience, also see Wikipedia's definition and overview at:
http://en.wikipedia.org/wiki/Neuroscience

Neuron, Vol. 40, 655–664, October 30, 2003
Both of Us Disgusted in My Insula:
The Common Neural Basis of Seeing and Feeling Disgust [pdf 194KB]
Bruno Wicker, Christian Keysers, Jane Plailly, Jean-Pierre Royet Vittorio Gallese and Giacomo Rizzolatti.