In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is an unwanted perturbation to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission. Signal noise is heard as acoustic noise if played through a loudspeaker; it manifests as 'snow' on a television or video image. Noise can block, distort, change or interfere with the meaning of a message in human, animal and electronic communication.
(strobist info: 430ex2 with snoot via canon 7d wireless)