Synesthesia

Synesthesia is a neurological condition in which stimulation of one sensory or cognitive pathway (for example, hearing) leads to automatic, involuntary experiences in a second sensory or cognitive pathway (such as vision).

Simply put, when one sense is activated, another unrelated sense is activated at the same time.

This may, for instance, take the form of hearing music and simultaneously sensing the sound as swirls or patterns of color.