The word anesthesia means "no sensation"; synesthesia means "joined sensation" (Greek, syn = together + aisthesis = perception). It refers to an involuntary physical experience in which the stimulation of one sense modality reliably causes an additional perception in a different sense or senses. For example, a synesthete might describe the color, shape, and flavor of someone's voice, or seeing the color red, a synesthete might detect the "scent" of red as well.

Synesthesia as a physical experience is distinct from the everyday cross-sensory associations familiar to [psychophysics], as well as from [metaphor] and [artistic contrivances] that sometimes use the word "synesthesia" to describe their deliberate multisensory joinings.

Although medicine has known of synesthesia for three centuries, it keeps forgetting that it knows. After decades of neglect, a revival of inquiry is under way. Neuroscience is particularly curious because of what synesthesia might tell us about consciousness, the nature of reality, and the relationship between reason and emotion.

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