Tones have Periodic (regular) oscillations, but Noise has Aperiodic (irregular) movements
A Pure Tone is also called a Sine Wave, because of the shape of the curve of the movement. It is also called sometimes a Simple Tone, because it represents a Single Frequency.
Hence, it is the smallest unit of sustained sound that one can have, as far as Tones are concerned. This, I suppose, means that there is another kind of sound other than a Tone. Yes, of course.
You remember how we described the movement of a molecule for a tone as being regular? Well you can have oscillations that are not regular.
My wife says that this describes my dancing ability perfectly. Above it is a graphic representation of an Aperiodic sound It is called a Noise.
NOTES: Some more information on Pure Tones.