Synesthesia from the ancient Greek (syn), "together," and (aisthēsis), "sensation," is a neurologically based condition in which stimulation

Stimulation

Stimulation is the action of various agents on nerves, muscles, or a sensory end organ, by which activity is evoked; especially, the nervous impulse produced by various agents on nerves, or a sensory end organ, by which the part connected with the nerve is thrown into a state of activity.The word...

 of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. People who report such experiences are known as synesthetes.

In one common form of synesthesia, known as grapheme → color synesthesia

Grapheme-color synesthesia

Grapheme → color synesthesia is a form of synesthesia in which an individual's perception of numbers and letters is associated with the experience of colors. Like all forms of synesthesia, grapheme → color synesthesia is involuntary, consistent, and memorable...

 or color-graphemic synesthesia, letter

Letter (alphabet)

A letter is a grapheme in an alphabetic system of writing, such as the Greek alphabet and its descendants. Letters compose phonemes and each phoneme represents a phone in the spoken form of the language....

s or numbers

Numerical digit

A digit is a symbol used in combinations to represent numbers in positional numeral systems. The name "digit" comes from the fact that the 10 digits of the hands correspond to the 10 symbols of the common base 10 number system, i.e...

 are perceived as inherently colored, while in ordinal linguistic personification

Ordinal linguistic personification

Ordinal-linguistic personification is a form of synesthesia in which ordered sequences, such as ordinal numbers, days, months and letters are associated with personalities...

, numbers, days of the week and months of the year evoke personalities.