Noise

Noise is any unwanted signal that appears in your sampled data. Noise comes from both external sources, such as the AC power line, motors, generators, transformers, fluorescent lights, soldering irons, CRT displays, computers, electrical storms, welders, and radio transmitters, and internal sources, such as digital clocks, microprocessors, and switched mode power supplies.

Noise can be transient in nature, have fixed frequencies such as harmonic or mixer products, or be broadband random noise.