A while back, I was trying to explain something to Tom related to the months of the year.  He clearly wasn't understanding what I was trying to describe, even though it seemed so clear to me.  After a few more experiences like this, he jokingly said to me one day: "I think you have synesthesia."

Never having heard the word before, I think I brushed him off ("No, YOU have synesthesia!) and continued my attempt to explain to him what I was seeing that he couldn't visualize.