Francis Galton first clearly described and documented synesthesia in the 19th century. The word "synesthesia" means mixed perception.
During its occurrence, it links the modalities with one another. This means that one type of sensory perception or conceptual idea provokes the conception of another sense (that has not been stimulated) in the conscience at the same time.
Therefore, a synesthete should be able to see music in colors or shapes in their conscience or to have their conceptions of numbers and letters be colored in different nuances.
Such a relation with color is most common, even though in rare cases, synesthetic images can contain olfactory, auditory and other conceptions.
The most accurate definition of synesthesia is "perception using color (or using another sensory perception) of some of the things that don't have color". Examples can be letters and numbers, but also music, emotional states and conceptions of other people.
The main and most commonly seen form of synesthesia is the linking of colors and graphemes (numbers and letters). It occurs most brightly in childhood. For most synesthetes, links occur between colors and graphemes, while some, besides these, have other mixed perceptions.
Synesthesia must not be confused with a disease. A synesthete is a person who perceives everything normally but along with this, possesses a synesthetic perception as well. Synesthesia is passed down genetically and can be characteristic for entire families.
For a person with such perceptions, the bases of the images are entirely inner conceptions. They are within his conscience and the synesthete perceives them as such only, and not as a part of reality.
For example, for a synesthete, the letter "V" is red. It is a specific red color. And they will always see it in this color, unwillingly.
The word "vagabond" for example will also be primarily red, due to the color of the first letter, but it will appear in their conscience with a shade of red different from that of V itself.
Whenever certain letters, words and numbers are actually written down, the synesthete will see them like anyone else - black on a white background without physically seeing their synesthetic color. It appears only when they think about them and imagine them written down in their conscience.