Refer to figure.
In trimmed level flight a disturbance in pitch usually oscillates about its original state causing fluctuations in the values of speed, height and indicated load factor. When an airplane has positive longitudinal dynamic stability the amplitude of the oscillations gradually diminish as the aircraft returns to its original trimmed flight state.

The oscillatory motion of the airplane in pitch after a disturbance to its trimmed state can be shown as two separate oscillations. One is a short-period oscillation and the other is a long-period oscillation called a phugoid. The phugoid is usually poorly damped; it has large variations of speed and height and has an almost uniform load factor (n). It has a constant energy motion in which the potential energy and the kinetic energy are continuously interchanging.

The motion that causes short-period oscillation is normally heavily damped. It not only involves small changes of speed and height but also large changes of load factor. The speed and height of the airplane remain approximately constant.