A Doppler shift toward higher observed frequency is called a blueshift. The term originally comes from visible light: increasing frequency moves a spectral feature toward the blue end of the visible spectrum.
Blueshift commonly occurs when a light source moves toward an observer, although the broader idea applies to any wave whose observed frequency increases. In astronomy, scientists measure shifts in spectral lines from stars and galaxies. The amount of displacement reveals the object’s motion along the line of sight, allowing astronomers to estimate radial velocity.
Blueshift is the counterpart of redshift, which indicates a lower observed frequency or longer wavelength. It does not automatically mean an object is physically blue, and it should not be confused with diffraction, which is the bending and spreading of waves around openings or obstacles. Also, not every blueshift is purely a simple Doppler effect: gravitational fields and other relativistic effects can alter observed light frequencies. In everyday quiz usage, however, higher frequency from relative approach is the canonical meaning.