1. Any unwanted signal. 2. A disturbance that does not represent part of a message from a specified source. 3. Sometimes restricted to random energy. To the extent the noise is random (but most noise is not), its amplitude can be attenuated by a factor of √(n) by compositing n signals from independent measurements. 4. Geologic noise is interference from unwanted geologic conditions. See also noise (electrical); noise (gravity and magnetic); noise (seismic).