Synesthesia rhymes with "anesthesia"
Anesthesia = no sensation
Synesthesia = joined sensation
In synesthesia two or more senses are automatically and involuntarily coupled such that a voice, for example, is not only heard, but additionally felt, seen, or tasted.
Synesthesia is not imagination or learning. It differs from metaphor and deliberate contrivances such as colored music. Some couplings are more common than others: sound–sight synesthesia (colored hearing) is plentiful, whereas taste and smell combinations are rare.
Some form of synesthesia occurs in 1 in 23 people. It runs strongly in families and is a trait—like having blue eyes—and not a disease. Synesthesia is no mere curiosity but a window onto how ordinary brains work. In fact, Richard Cytowic says we are all synesthetic, except we don’t know it.