Real Dictionary

blueshift in American English
the shift of spectral lines toward the shorter wavelengths and higher frequencies at the blue end of the spectrum of a luminous celestial body, indicating that the light source is moving rapidly toward the observer
noun: a shift toward shorter wavelengths of the spectral lines of a celestial object, caused by the motion of the object toward the observer

blueshift in British English
noun: a shift in the spectral lines of a stellar spectrum towards the blue end of the visible region relative to the wavelengths of these lines in the terrestrial spectrum: a result of the Doppler effect caused by stars approaching the solar system