Synesthesia is when two senses combine with each other and form relationships. For example, I have the most common kind: grapheme-color synesthesia. Mine looks like this. The only one that’s supposed to be black is “1”. Otherwise, the ones in black have no color associated with them.
Yeah I didn’t realize it wasn’t normal until late high school, when I asked my best friend what her color associations were, and she just didn’t have a clue what I was talking about.