What is the meaning of x in X-ray?

XXX, XXXX, XXXXX

[fôreks′, fīveks′]

the designations for abnormal sex chromosome complements in the human female in which there are, respectively, three, four, or five instead of the normal two X chromosomes so that each somatic cell contains a total of 47, 48, or 49 chromosomes. Although there is no consistent phenotype associated with such aberrations, the risk of congenital anomalies and mental retardation in the affected individual increases significantly as the number of X chromosomes increases.

Ref. Mosby's Medical Dictionary, 8th edition. © 2009, Elsevier.