(electromagnetic radiation, wavelength 0.01 to 10 nanometers)
The term X-ray generally refers to electromagnetic radiation (EMR) with a wavelength
within the range of 0.01-10 nm, lying
between ultraviolet and gamma rays.
Lower energy/frequency X-rays, toward the ultraviolet end,
are termed soft (SXR) and higher energy/frequency, toward the
gamma rays end are termed hard (HXR).
Intervening thresholds chosen to distinguish
soft and hard are typically in the general region of 0.1-0.2 nm or 5-10 keV.
Rather than by frequency or wavelength, X-rays are often
cited by photon energy, in electron volts (eV).
The equivalent to the above-cited range
for X-rays is 124 eV to 124 keV, but astronomers often cite the line
between X-rays and gamma rays as 100 keV:
this difference is largely moot as in discussions of this EMR,
a specific photon-energy range of interest is likely to be stated.
There is not a consensus across all sciences and industries on the
boundaries of EMR termed X-rays,
especially between X-rays and gamma rays for which
the distinction originally arose from what
produced them: X-rays were from high-voltage vacuum tubes
producing cathode rays, and gamma rays are a type of
emission occurring during radioactive decay
of radioactive substances such as radium. However, both these types
of sources are producing EMR, and the respective wavelength
ranges overlap,
an example of the fact that mechanisms that produce EMR
don't adhere to any particular absolute limits.
In astronomical observation, any possible boundary would still be
spanned by some instruments and some sources, but using some
unambiguous classification-criteria is convenient even if it
is arbitrary.
X-rays from space (like gamma rays) are virtually blocked by Earth's
atmosphere, helping make Earth habitable, and X-ray telescopes
are space-based.
Among the telescopes/observatories:
Also, gamma-ray and UV observatories often have some X-ray capability,
as well as solar observatories and some planetary probes.
Some past X-ray telescopes/observatories: