the easiest way to understand synesthesia is that its a bran problem where a person uses more than one of their senses at a time. its not always about colors and numbers.

my MS caused me to have this. I may look at an apple but at the same time as my eyes are seeing the apple my mouth has the taste of an apple, and at the same time as my eyes are seeing the apple and my mouth is tasting the apple, I hear the sound of an apple crunch in my ear, and can spell the apple.

this is all going on with out my even taking a bit of the apple, just sensory hallucinations because I am looking at an apple.

this sometimes gets more compounded if for example the apple is red on a dark colored bowl and the bowl is sitting on a bright table.

my senses do an optical illusion kind of thing where the apple color will blink from red to dark color of the bowl to the bright color of the table.

my point is that big word synesthesia just means your brain is processing information from more than one side of things at the same time, instead of like other people do, which causes sensory hallucinations. (not psychosis kind of hallucinations)