Synesthesia. Essentially it means the "crossing" of two or more senses. The more I read about it, the more common researchers say it is, but it's still less than 5% of the population.
For me, it means sounds have colours. If you wanna know more about Synesthesia as a phenomenon, I recommend Karen W's website on the subject, followed by a quick google of the many synesthesia-related resources on the web.
You can also email me, but I'm no expert. My knowledge is of my own experience, and what I have read about. If you email me to ask about Synesthesia, replies depend upon my mood. If you wish to make any inquiries about my artwork, where you can see more, how you can purchase some etc.. reply guarenteed :D