What is AWGN (Additive White Gaussian Noise)?

Additive White Gaussian Noise is a basic noise model. Additive because it is added to any system whether intrinsic or extrinsic. White refers to the idea that it has uniform power across the frequency band for the information system. Gaussian because it has a normal distribution in the time domain. Thermal noise in electronic systems is usually modeled as AWGN.