A Pure Tone occurs when a molecule oscillates regularly and smoothly back and forth.
A single molecule oscillating up and down as a result of a noise would leave a tracing of its movement as a straight line.
If a more steady sound lasted for a second while I pulled the monitor screen to the left, the tracing of the movement of the molecule might look like this:
This is a description of how this molecule moves in time--smoothly and regularly (Periodic). This one would be called a Pure Tone.