Synesthesia is when one of your senses can stimulate another sense or cognitive path in your brain. So where others might just see a blue color, someone with synesthesia might see blue and additionally hear a ringing sound or think of the number three.
After watching a recent episode of Dr. Who, it made me think that it would be sorta neat if some artists (like Van Gogh) actually had synesthesia and that their paintings could only truly be appreciated by someone that suffered from the same condition. A painting of sunflowers might also convey a funeral dirge or an image of a starry night might taste like vanilla.
What does this have to do with RPGs? Well, back in 2ed (or was it 3e?), the psionics handbook had one power that granted a similar ability (I think they also called it synesthesia which prompted me to look up the definition in the first place). It was a way for a psion to deal with blindness or avoid gaze attacks by allowing them to “see” with their ears or sense of touch.
Actually, it is known (or at least guessed…) that some artists had synesthesia. I don’t remember who, but I know I read it somewhere. It can also be induced by certain drugs (such as LSD).
Also, from what I’ve read, it can also go along with music. They hear the song and see colours and stuff associated with each note/chord. Similar to the def girl on season whatever of Heroes.