X chromosome
         n 1: the sex chromosome that is present in both sexes: singly in
               males and doubly in females; "human females normally have
               two X chromosomes"

English Dictionary: X-Chromosom-Inaktivierung	by the DICT Development Group
From WordNet (r) 3.0 (2006) [wn]:
X-ray machine n an apparatus that provides a source of X rays
From WordNet (r) 3.0 (2006) [wn]:
X-raying n obtaining images by the use of X rays [syn: X-raying, X-radiation]
From Webster's Revised Unabridged Dictionary (1913) [web1913]:
Xeronic \Xe*ron"ic\, a. [Gr. [?] dry + citraconic.] (Chem.)       Pertaining to, or designating, an acid, {C8H12O4}, related to       fumaric acid, and obtained from citraconic acid as an oily       substance having a bittersweet taste; -- so called from its       tendency to form its anhydride.