Irregular level fluctuations of a low order of magnitude. All analog video signals contain random noise. Ideally for digital sampling, the noise level should be lower than the accuracy of the sampling, i.e. less than one LSB of the digital dynamic range. Pure digitally generated signals however have no noise, a fact that can be a problem under certain conditions.
With digital compression, noise has a new importance. Noise, which can originate from analog sources, can be hard to distinguish from real wanted high frequency information. This means compression coders can waste valuable output bandwidth describing the noise at the cost of the the quality of the digital pictures or sound.
See also: Dynamic Rounding