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The frequency of a simple light wave is directly related to its colour (cf. spectrum). For the highest possible frequencies, the colour is blue-violet. If the frequency of a light wave is shifted towards higher frequencies (for instance by the doppler shift), that corresponds to a colour shift towards the blue-violet end of the spectrum, and is hence called a blue-shift.

From this, "blue-shift" has come to acquire a more general meaning. It is used to denote any shift towards higher frequencies, even for types of electromagnetic radiation where the frequencies do not correspond to any visible colour, and more generally still, for other types of waves as well (for instance for gravitational waves).