What is the broadening of spectral lines caused by velocity-related Doppler shifts called?

The broadening of spectral lines caused by velocity-related Doppler shifts is called Doppler broadening.

A spectral line represents radiation emitted or absorbed at a characteristic frequency or wavelength. In a real gas, atoms and molecules are not all moving at the same speed along the observer’s line of sight. Particles moving toward the observer produce slightly higher frequencies, while particles moving away produce lower frequencies. The combined shifts spread a narrow line into a broader profile.

Thermal Doppler broadening is the most familiar case. Hotter material has a wider distribution of particle velocities, so its spectral lines become wider. For a given transition, lighter particles also move faster on average than heavier ones at the same temperature. The resulting thermal profile is approximately Gaussian, and its width can help astronomers and physicists estimate temperature.

Doppler broadening is not the same as Zeeman splitting, which arises from magnetic fields, or Stark broadening, which arises from electric-field effects and collisions. Pressure broadening is another distinct mechanism. Several mechanisms can act together; thermal Doppler and pressure broadening, for example, can combine into a Voigt line profile.