X

X, 24th letter of the English alphabet, corresponding to the 21st letter of the Latin alphabet, which was itself derived from a letter of the western subdivision of the ancient Greek alphabet representing the sound “ks.” The same letter in the eastern Greek alphabet represented “ch” or “kh,” and as such passed into the later Greek and Cyrillic alphabets. X represents 10 in Roman numerals, and in algebra and in the sciences it is used as the symbol for an unknown quantity.