Concept question - What is noise? I've been studying image processing methods (without signal processing background) and many things are interesting to me. I was think about noise. In many definitions noise is unwanted  and without meaning  electrical or electromagnetic energy that degrades the quality of signals and data. Considering noise a signal type. So my question is 

Noise is undesirable signal, or incomprehensible signal?

Note: Sorry by the philosophical question.
Noise is definetly undesired, but not incomprehensible.

The fact that noise is comprehensible is the reason we talk about it. Cosmic Rays might be present, but they have no effect on an old mechanical clock. Would you consider them "noise" when you read the clock?

You might call that loud disturbing sound that you hear "noise". A deaf person would not consider that noise, because it's incomprehensible.