Vibration is simply defined as the variation of an object about a fixed position. To properly define and quantify the physical characteristics of vibration, however, is much more complex – account must be taken of the direction, frequency and magnitude of vibration. Then, what properties should be measured? Is it appropriate to measure the displacement of the vibrating object as a measure of its amplitude, or should we instead measure velocity, or acceleration? What are the conventions for measuring direction, and is it really important?