A blueshift is any decrease in wavelength (increase in frequency); the opposite effect is referred to as redshift. In visible light, this shifts the colour from the red end of the spectrum to the blue end. The term also applies when photons outside the visible spectrum (e.g. x-rays and radio waves) are shifted toward shorter wavelengths, as well as to shifts in the de Broglie wavelength of particles. Blueshift is most commonly caused by relative motion toward the observer, described by the Doppler effect. An observer in a gravity well will also see infalling radiation gravitationally blueshifted, described by General Relativity in the same way as gravitational redshift. In a contracting universe, cosmological blueshift would be observed; the expanding universe gives a cosmological redshift, and the expansion is observed to be accelerating.
Source: wikipedia, NASA