Synesthesia: A neurological condition in which an object or concept perceived by one sense is accompanied by a perception in a different sense or in which a perception manifests as an additional perception in the same sense.

For example, a synesthete might contend that colors possess musical tones or that a letter is always a certain color.

An aria’s the loveliest thing that I have ever seen
And when I go aswimming the water tastes like green
And though you may say tis only in my head
I always think of Wednesday as being darkest red