Synesthesia is the experience of one type of stimulus through an unrelated sense. Certain hallucinogenic drugs, neurological conditions, and even sleep deprivation can cause one’s subjective experience to be altered in this way.
Synesthetic imagery is also used as a poetic gesture; when Bob Dylan sings in Visions of Johanna that “the ghost of ‘lectricity howls in the bones of her face,” he’s signaling that something more is going on than the simple visual effects of “lights flicker[ing] from the opposite loft.”
As a species, we seem to like having our senses appealed to in unlikely ways. I was most recently impressed by this upon seeing composer Aleksei Stevens’ account of his latest composition Study no. 1 for amplified acoustic guitar. It’s his “first real foray into…graphic scores”, and it’s gorgeous just to see. Strictly speaking, this is not synesthesia; it’s the graphical representation of instructions to the performer. It’s somewhat separate from the piece as it will be performed. However, it’s possible – as with any score – to look at the piece and begin to hear what the composer has rendered.
Update: The first graphic score I encountered was John Cage’s Aria. That guy knew his mushrooms.