X is for X-ray! (Also, x ray, X ray.) noun
a. Often, x-rays. a form of electromagnetic radiation, similar to light but of shorter wavelength and capable of penetrating solids and of ionizing gases.
b. such radiation having wavelengths in the range of approximately 0.1–10 nm.
2. a radiograph made by x-rays.
3. ( initial capital letter ) a word in communications to represent the letter X.