Oscillation (9)
I believe the answer is:
vibration
'oscillation' is the definition.
(I know that vibration is a type of oscillation)
This is all the clue.
(Other definitions for vibration that I've seen before include "Palpitation, oscillation" , "Quick back and forward movement" , "Pulsation" , "Rapid to-and-fro motion" , "Oscillating movement" .)