Word of the day: synesthesia
Synesthesia is a perceptual phenomenon in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. Synesthetic associations can occur in any combination and any number of senses or cognitive pathways.
In one common form of synesthesia, known as grapheme-colour synesthesia, numbers are perceived as inherently coloured. In spatial-sequence or number form synesthesia, numbers, months of the year, or days of the week elicit precise locations in space.
You can learn more about this condition here.
I recently came across the term while watching a fascinating television series about the brain described here.