If the X-rasy coming from a coolidge tube are examined for wavelengths present, and the intensity of different wavelengths are measurea and plotted, we can observe that there is a minimum wavelength below which no X-ray is emitted.
The wavelength below which no X-rays are emitted is called the cut-off wavelength or the threshold wavelength.
From the plot it can also be observed that at certain sharply defined wavelengths, the intensity of X-rays is very large. These X-rays are called characteristic X-rays.
At other wavelengths the intensity varies gradually and these X-rays are called continuous X-rays.
X-rays emitted due to electronic transition from a higher energy state to a vacancy created in the K shell are called K X-rays.