X, 24th letter of the alphabet. In English it has no peculiar sound, but stands for the combination ks as in fox, or gz as in exempt, or, initially, for the sound of z as in xenia. In words from Greek, x transliterates the 14th Greek letter, xi, a letter perhaps quite unrelated to the Roman x in form. The formal Greek correspondent of x was chi, hence it is used in phonetics to represent a velar fricative like ch in loch. As the initial of the name Christ, X has become a symbol for it, e.g., in Xmas and in the monogram XP (chi rho). In Roman numerals X stands for 10.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.