GNU Collaborative International Dictionary of English

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Found one definition

1.       
   X . X, the twenty-fourth letter of the English alphabet, has three sounds; a compound nonvocal sound (that of ks), as in wax; a compound vocal sound (that of gz), as in example; and, at the beginning of a word, a simple vocal sound (that of z), as in xanthic. See Guide to Pronunciation, §§ 217, 270, 271.
   1913 Webster
    The form and value of X are from the Latin X, which is from the Greek Χ, which in some Greek alphabets had the value of ks, though in the one now in common use it represents an aspirated sound of k.
   1913 Webster