Synesthesia - from the Ancient Greek σύν (syn), ‘together,’ and αἴσθησις (aisthēsis), ‘sensation’—is a neurologically-based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway.
(via xn—7xa)
I have several friends with synesthesia, and I may have a very mild touch of it myself (sounds have colors to me, but I am able to ignore them most of the time).