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Red noise is a term for randomness in signals that is not totally uncorrelated (which would be white noise) but has a particular frequency distribution with more of the lower frequencies (so-named because the color red comprises the lower frequencies of visible light). The term is often used specifically for the case where the power at a particular frequency is related to the reciprocal of the square of the frequency (1/f²). It is also called Brown noise or Brownian noise because Brownian motion matches it. The term is used in discussions of signal detection. One example is the detection of extra-solar planets through astrometry, for which measurement noise is due to equipment and atmospheric anomalies.
There are various analogous terms for random noise with various frequency distributions, such as pink noise, blue noise, and violet noise:
| term | tendency | relation to frequency |
| white noise | same regardless of frequency | constant |
| pink noise | falls with frequency | ~1/f |
| red noise | more pronounced fall | ~1/f² |
| blue noise | rises with frequency | ~f |
| violet noise | more pronounced rise | ~f² |