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.
For me, absolutely everything has a color. Numbers, letters, sounds, days of the week, months… All of it. I didn’t know this wasn’t normal until I was well into my teens.