1. any unwanted sound or, more generally, any unwanted disturbance (e.g., electrical noise).

2. a random or aperiodic waveform whose properties are described statistically. There are many types of noise, which are distinguished by their spectral or statistical properties. White noise has equal energy at all frequencies; broadband noise has energy over a relatively wide frequency range (e.g., 50 Hz to 10 kHz for audition); pink noise has energy that is inversely proportional to frequency; and Gaussian noise has instantaneous values that are determined according to a normal probability density function.

3. anything that interferes with, obscures, reduces, or otherwise adversely affects the clarity or precision of an ongoing process, such as the communication of a message or signal.