What “X” might stand for

The crossword question “What ‘X’ might stand for is TEN” is referring to the Roman numeral system, in which “X” represents the number ten. In this system, letters are used to represent numbers, with “I” representing one, “V” representing five, “X” representing ten, and so on. Therefore, if you see the letter “X” in a Roman numeral, you know that it is representing the number ten.