In general: Unwanted part of data superimposed on a wanted signal. Noise is often modeled as a stationary random process, but may also contain signal-dependent components. One man's noise can be the other man's signal. More specific in seismology: Incoherent natural or artificial perturbations caused by a diversity of agents and distributed sources. One usually differentiates between ambient background noise and instrumental noise. The former is due to natural (ocean waves, wind, rushing waters, animal migration, ice movement, etc) and/or man-made sources (traffic, machinery, etc.), whereas instrumental (internal) noise may be due to the "flicker" noise of electronic components and/or even Brownian molecular motions in mechanical components. Digital data acquisition systems may add digitization noise due to their finite discrete resolution (least significant bit). Very sensitive seismic recordings may contain all these different noise components, however, usually their resolution is chosen so that only seismic signals and to a certain degree also the ambient noise are resolved. Disturbing noise can be reduced by selecting recording sites remote from noise sources, by underground installation of seismic sensors (e.g., in boreholes, tunnels or abandoned mines) or by suitable filter procedures (improvement of the signal-to-noise ratio). See Chapter 4 of this Manual, Webb (2002), and Wielandt (2002)