This is a term used for watching the dynamic behaviour of a system during it's normal mode of operation. In vibration terms the motion is normally measured using accelerometers in the time domain and then Fourier Transformed into the frequency domain so that the motion at certain frequencies may be observed. This frequency may be related to a certain order of rotation in the case of an engine or electric motor. Where many measurement points are required to accurately describe the motion of a system it may be necessary to make a number of measurements. In this case the phase relationship between measurements may be discerned by using a reference transducer fixed at one location on the structure to which all other phases are measured relative.