Neurologically based misperception or “cross-wiring” of sensory modalities. Individuals with synesthesia might perceive letters or numbers as possessing inherent colors, or they might associate colors with certain musical tones.
Example: A patient reports that environmental sounds cause his skin to tingle, and that being touched causes him to see fireworks.
Grinnell, R. (2016). Synesthesia. Psych Central. Retrieved on September 23, 2017, from https://psychcentral.com/encyclopedia/synesthesia/