Synesthesia or Synaesthesia is a very interesting phenomenon. It is a condition of the mind resulting when the brain’s processing centers of the senses are interconnected. People who “suffer” (or better benefit) from it, experience the world in a very different and unique way. They are called synesthetes.
They can “hear” or “taste” colors, they see colored letters, they can “see” sounds, and they perceive a sensation with more than one senses. For example, a smell can engage their vision or any other sense. They are not hallucinating they are just synesthetes.
This would be very interesting. People do not generally suffer from this condition, as it is a condition that greatly enhances your memory and problem solving ability above a person without the condition.