A vacuum is a volume of space that is substantively empty of matter so that gaseous pressure is much less than standard atmospheric pressure.
A vacuum is used in the x-ray tube because otherwise the electrons would be stopped by air or any other gas. Unlike in closed (or sealed) x-ray tubes, in which the vacuum is permanent, open tube systems have to be outfitted with a vacuum pump for the purpose of creating vacuum during tube warm-up.

The x-ray (or roentgen-ray) spectrum consists of electromagnetic radiation with wavelengths shorter than ultraviolet (UV) and longer than gamma rays. The usual photon energies of x-rays range from 100 electron volt (eV) to 100 keV (wavelengths of around 10 to 0.01 nanometers; or around 100 to 0.1 Angstroms); corresponding to frequencies in the range of 30 PHz to 30 EHz (see Hertz).
The energy distribution (wavelength, frequency) of x-ray photons emerges from the source, the x-ray tube. In a conventional tube, x-rays are generated in two different ways that, together, form a typical spectrum consisting of the bremsstrahlung, which is superimposed by the lines of the characteristic spectrum (in a graph, the curve is shaped like a hump topped by several spikes).

See also Angstrom, Direct Radiation, Secondary Radiation, and Radiation Meter.