Synesthesia is an incredibly rare experience where your brain has multiple sensory responses to an unrelated stimulus. People who have these experiences are known as synesthetes. In some cases or this condition, letters and numbers can induce a certain color in one's mind or hearing certain sounds can induce a color sensation. This experience is caused by a neurological condition that stimulates many different senses. Although it was not officially called "synesthesia", the condition is found consistently in throughout history.
I personally have not met a synesthete but I have so many questions about what the experiences are like. We have grown up with certain phrases like "feeling blue" when you are sad and "green with envy" when you are jealous but I was wondering if we were given these comments by someone who was a synesthete a long time ago. Since there is such a broad history of synesthesia, I like to think that maybe some of the common phrases and ideals we know were passed down by generations of past synesthetes.