Pink noise (AKA 1/f noise or flicker noise) is similar to white noise except that it contains an equal amount of energy in each octave band. To put it technically, the power spectral density is proportional to the reciprocal of the frequency.
Hear a 5-second example of pink noise (download file).
Sound engineers use pink noise to test whether a system has a flat frequency response.
Pink noise can be generated by putting white noise through a pinking filter which removes more energy as the frequency increases (approximately 3 dB per octave).
As white noise is anagous to white light (representing all frequencies equally), pink noise is anagous to light which tends towards the lower end of the visible light spectrum (red light).
Main page: Noise Colours