A condition where real information from one sense is joined with false perceptions from other senses. People with this condition can "see" sound, or "smell" colors, etc. The experience varies from person to person. One synesthetic may have a solid color overlaying their vision, corresponding to pitch and volume; another may see different colors for different letters (K is always red, C is always blue for example). Synesthesia is often experienced when taking high doses of LSD; the likelihood of this effect appears to directly correlate to dosage.
Synesthetics often become composers or musicians; their unique ability to visualize sound is a distinct advantage.
A condition which causes the senses to be crossed wired. For example: numbers, letters, words, music can all have colors, shapes, personalities, feelings. The equivalent of being born on LSD. Introverts are more likely to have it than extroverts.
Because of synesthesia; the number 4694 looks like beautiful fall colors to me.