Noise in a system that is produced by the system itself. In electronics all components produce their own noise so long as they are at any temperature above absolute zero. If electrons are moving there is noise. Add the noise of the components and the circuits they are in that are part of the audio path together and you have the self-noise of the device. Similarly, when you combine many such devices together in an audio system you have the self-noise of the system. This self-noise represents the noise floor of the device or system. The difference between this noise level and the level of a signal in the device is the signal to noise ratio.