From the dictionnary, synesthesia is a phenomenon of two senses being combined, where one stimulus awakes two senses instead of the unique one it should target.
For example, as is the case for Alexandra Graff in Concerto in Chroma Major, sounds turn into colors and shapes, which is a form of Synesthesia referred to as Chromesthesia.
Naturally, that perception varies from one synesthet to the next, but will be constant for each individual–a particular note, or timbre, will always have the same appearance over time, which can lead to sentences such as :
Darren Criss has a very bubbly, purple singing voice, don’t you think?
And if you’re not a synesthet, that sentence may make little to no sense at all.
The way I see it, since Alexandra’s Synesthesia is a part of my own personal experience #ownvoice, Synesthets don’t always realize that something is different about the way they apprehend the world around them. Seeing colors when listening to sounds is as natural as any other sensation. It takes expressing that sensation to someone “unaffected” for the truth of Synesthesia to come to light.
“Oh, not everybody sees music and voices the way I do. Ooooh?”
Usually followed by, “what is wrong with me”, since the reaction from non-synesthets is often one of shock and doubt.
This does explain why so little research has been done around Synesthesia, and there is a whole world beyond the five senses still in need to be explore!