Synesthesia in its original sense refers to a secondary response to stimuli in which the original sensation has an added element. The best known form of the basic synesthetic is grapheme/synesthesia, in which written characters are perceived as colored, when written in black and white.
This is a sort of sensory association, but there are psychological forms of synesthesia, including a form which involves numbers and dates being associated with personality types.
Synesthesia is an involuntary physiological or psychological response to these stimuli. Tests have shown major differences in brain activity between syesthetes and basic normal brain patterns. The cause is believed to be a result of functions in various regions of the brain. One possibility is a difference in the brain's 'wiring' between individuals, with synaptic variables causing synesthesia. About 60 forms of synesthesia have been identified.
Synesthesia is associated with some learning difficulties. On the positive side it's also linked to creative art.