I have mild synesthesia for letters, numbers, shapes and scale degrees. At least in my case (and this is wildly personal and subjective) I'd agree that synesthesia basically is association. It's kindof hard to put into words, like for example

> Apple

I can see plain as day that the text above is black font on a light tan background. But the "A" is obviously yellow, each "p" is pink, the "l" is lightish grey, and the "e" is green. The word as a whole takes on the yellow from the A it begins with, but is tinted based on the other sounds it contains. I don't "see" these supplementary signals so much as strongly feel them, like it's a side effect of the language processing that's going on when I read.

I don't know the underlying mechanics that trigger this association, but it's quite consistent. I'm guessing that it's a side effect of how my memory of language is organized, some processing trick my mind is using to make the storage more efficient. I was surprised to learn later in life that this is unusual, like, "doesn't everyone see the association in their mind's eye like this?" Apparently not. :)

The mind is a fascinating thing.