Odds are high the twenty-fourth letter of the alphabet will offer a good substitute if you don’t know something.
Wilhelm Röntgen didn’t know what to call a newly discovered form of radiation, so he named them x-rays. The world didn’t know what to call the generation born between the mid-1960s and the late-1970s, so it was named Generation X.
In algebra you are surrounded by them. They come at you from the left, from the right and if you study too late at night they invade your dreams carrying pitchforks.
If you have ever wondered about this mystery letter of unknown then take a gander at this short TED talk.
Enjoy!