X

CHI

This Greek letter is the 22nd in the alphabet and often represents a statistical variable or the ratio of a circle's circumference to its diameter.

"X" often represents the Greek letter chi, which looks like an X and is pronounced "kai" (or "chi" in English). In crosswords, single-letter clues like this usually point to Greek letters, and chi is the 22nd letter of the Greek alphabet. It’s a straightforward abbreviation where the symbol stands in for its name.