What is Synesthesia?

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."

http://faculty.washington.edu/chudler/syne.html

Also see:

http://en.wikipedia.org/wiki/Synesthesia