An oscillation is a projection of a circular motion to a lower dimensional space. The delay is called phase lag. If the phase lag is pi/2, then it is also calle

An oscillation is a projection of a circular motion to a lower dimensional space. The delay is called phase lag. If the phase lag is pi/2, then it is also called derivative where one quantity measures the rate of change in the other. The quantities are running around in a circle with a phase lag.