(n.) 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.

dark horse enigma frontier frontiers of knowledge matter of ignorance mistake mystery n puzzle riddle sealed book terra incognita the incalculable the strange the unfamiliar the unknowable the unknown unexplored ground unexplored territory unknown quantity z