While “synesthesia” meaning “union of senses” implies the association of two sensory elements with little connection to the cognitive level, empirical evidence indicated that most phenomena linked to synesthesia are in fact induced by semantic representations. – from Wikipedia
There are two overall forms of synesthesia:
projective synesthesia: people who see colors, forms, or shapes when stimulated (the widely understood version of synesthesia).
associative synesthesia: people who feel a very strong and involuntary connection between the stimulus and the sense that it triggers. – from Wikipedia