Blue Shift refers to a shortening of a transmitted signal's wavelength, and/or an increase in its frequency. The name comes from the fact that the shorter-wavelength end of the optical spectrum is the blue (or violet) end, hence, when visible light is compacted in wavelength, it is "shifted towards the blue", or "blue-shifted". Since the longer-wavelength end of the visible electromagnetic spectrum is red, the opposite effect, of a lengthening of a signal's wavelength, is referred to as redshifting.
These terms and conventions ("blue" = compaction, "red"=stretching) are used even when referring to signals outside the optical range (for instance, radio waves, x-rays and gamma rays).
These effects are usually associated with relative motion between the signal source and the observer (a Doppler shift or Doppler effect), where a blueshift will be caused by the two moving towards each other. The term is also used informally to refer to a hypsochromic shift in photochemistry.
