Generally, a signal containing such a broad frequency spectrum that it is impossible to discern a particular pitch. Noise usually produces a hissing, crackling, or rumbling sound depending on how it is filtered. The two most common noise circuits usually found in synths produce white noise and pink noise. Other types of noise are defined, and are usually labeled (rather arbitrarily) with the name of a color, such as brown noise. See this Wikipedia article for more information.