The idea of noise is now widespread in many fields of study. However to a large extent the use of this term is unexamined. It has become part of the practice of science without entering to a significant extent as part of its explicit theory. Here I try to produce a clearer and more coherent account of the term. I start with a picture of noise from electrical engineering. I then generalise this to the widest conception: that of noise as what is unwanted. A closely related conception is noise as what is unexplained. A particular case of this later usage is where a source of randomness can be used to stand-in for this residual. I argue that noise and randomness are not the same. I explore the possible relation between noise and context, and propose a new conception of noise: namely that noise is what can result from an extra-contextual signal. I finish with an application of the analysis of noise to the relation of determinism and randomness.