Is defined as being a random fluctuation in voltage or current. Some sources of noise are related to known physics while others have no understood relationship.
Since noise is random, you cannot predict the actual waveform and analysis must be treated using statistical means. For noise sources that are described as white, the power spectrum of the noise is flat across a wide range of frequency. Those defined as pink noise have more intense noise at lower frequency.
There are several types of noise that are present in semiconductors. They fall into the classes of device noise, manufacturing related noise and environmental noise.