Xi is the 14th letter of the Greek alphabet, falling between nu and omicron. In English, it's pronounced something like "sigh."
You may let out a sigh of relief when you make this play in Scrabble, because it clears your rack of the "x," which will subtract eight points from your score if you're stuck with it in the final tally. So keep "xi" in mind. It's rare that you can't find a place to squeeze this little word in, dispose of the "x," and pick up nine points in the process.