X1  n. (also x) (pl. Xs or x's) 1 twenty-fourth letter of the alphabet. 2 (as a roman numeral) ten. 3 (usu. X) algebra first unknown quantity. 4 unknown or unspecified number or person etc. 5 cross-shaped symbol used esp. To indicate position (x marks the spot) or incorrectness, or to symbolize a kiss or a vote, or as the signature of a person who cannot write.