What is Synesthesia?
Synesthesia is a condition of mixed sensations: a stimulus in one sensory modality (e.g., taste) involuntarily elicits a sensation/experience in another modality (e.g. vision). Likewise, perception of a form (e.g., a letter) may induce an unusual perception in the same modality (e.g. a color).
This video gives a good explantion: