It's kind of hard to explain, so here's the definition:
Synesthesia is a condition in which one sense (for example, hearing) is simultaneously perceived as if by one or more additional senses such as sight. Another form of synesthesia joins objects such as letters, shapes, numbers or people's names with a sensory perception such as smell, color or flavor. The word synesthesia comes from two Greek words, syn (together) and aisthesis(perception). Therefore, synesthesia literally means "joined perception."
It's estimated that 1 in 200 or 1 in 100,000 people have it, though many more may not realize it.
Dan was one of them.
I first became interested in the condition when I was reading Nabokov's memoir, "Speak, Memory" and writing a paper on it in graduate school. Nabokov also had this condition.
Then Dan and I saw a special on it a few years later on 20/20 or PBS- one of those. And as we talked about it more- that's when Dan said,
"Oh, I have that. When I hear a music note, I see a color."
I was excited and proceeded to test it out. I'd say the name of a note- Dan would then hear it in his head and tell me the color. They were specific colors I remember- like a greyish purple or a bright orange. It was fascinating.
Biologically, it is believed that the cross-wiring that causes synesthesia is present in all babies at birth. Later as children grow, they lose these connections. This makes sense to me- you and I shared a child-like view of the world- and more than anything else- music, faith, attraction- I believe this is the shared bond that brought us together.
All info on synesthesia came from here: http://faculty.washington.edu/chudler/syne.html