Wiktionary
n. A signal or process with a frequency spectrum such that the spectral energy density is proportional to the reciprocal of the frequency.
Wikipedia
Pink noise or noise is a signal or process with a frequency spectrum such that the power spectral density (energy or power per Hz) is inversely proportional to the frequency of the signal. In pink noise, each octave (halving/doubling in frequency) carries an equal amount of noise energy. The name arises from the pink appearance of visible light with this power spectrum.
Within the scientific literature the term pink noise is sometimes used a little more loosely to refer to any noise with a power spectral density of the form
$$S(f) \propto \frac{1}{f^\alpha}$$
where f is frequency and 0 < α < 2, with exponent α usually close to 1. These pink-like noises occur widely in nature and are a source of considerable interest in many fields. The distinction between the noises with α near 1 and those with a broad range of α approximately corresponds to a much more basic distinction. The former (narrow sense) generally come from condensed matter systems in quasi-equilibrium, as discussed below. The latter (broader sense) generally correspond to a wide range of non-equilibrium driven dynamical systems.
The term flicker noise is sometimes used to refer to pink noise, although this is more properly applied only to its occurrence in electronic devices. Mandelbrot and Van Ness proposed the name fractional noise (sometimes since called fractal noise) to emphasize that the exponent of the spectrum could take non-integer values and be closely related to fractional Brownian motion, but the term is very rarely used.
Usage examples of "pink noise".
There's an audible hiss of pink noise as his glasses whisper in his ears, bone conduction providing a serial highway to his wetware.
When she said it, Wednesday suddenly realized that she could: the pink noise field was like tinnitus, scratching away at the edges of her implant perceptions.
Now the noise increased, became an increasingly louder rushing noise, like white or pink noise gradually increasing in volume to nearly unbearable levels.