To be able to describe a vibration two parameters are necessary. First we need to know the frequency at which the vibration cycle is repeated. A cycle is one complete oscillation, radio engineers often refer to this as the wavelength λ - (lambda). The second parameter is the size or amplitude of the oscillation, these are shown in Figure 1.

Figure 2 illustrates a pendulum which has drawn a sine wave trace on a strip of paper moving behind it. It has taken 3 seconds to complete 5 full cycles as measured from the reference position. The frequency in this case is 1.666 cycles per second. The term cycles per second is often referred to as Hertz or shortened to Hz.

The pendulum illustrated is generating a low frequency vibration with large amplitude, as a consequence the amplitude may be measured in terms of inches or millimetres of displacement. Higher frequencies with smaller amplitudes are better measured using either velocity or acceleration.

The example in Figure 2, for simplicity, ignores the fact that the amplitude of the pendulum will naturally decay to zero over a relatively short period of time.