It is the motion of a mechanical part back and forth from its position of rest / neutral position at certain frequency.
Amplitude is the size of the vibration signal. The amplitude of the vibration signal determines the severity of the fault. The higher the amplitude, the higher the vibration, the bigger the problem.
Amplitude depends on the type of machine and is always relative to the vibration level of a “good”;“new” machine! Frequency is the number of times an event occurs in a given time period (the event being one vibration cycle). The frequency at which the vibration occurs indicates the type of fault. That is, certain types of faults “typically” occur at certain frequencies. By establishing the frequency at which the vibration occurs, we get a clearer picture of what could be causing it.