From ActuallyAutistic Wiki
Synesthesia ("synaesthesia" in English English) is when you perceive information intended for one sense with an additional bonus sensation in another.
Common examples include seeing colours when hearing sounds or seeing numbers. Less common examples include ticker tape synesthesia, which involves scrolling subtitles for your thoughts and/or people's speech. (Alas, this probably doesn't help out those of us who also have auditory processing disorder.)
There are some great resources on synesthesia. I am a synesthete and can eventually add a summary here.