The tendency of an airplane to pitch up and down over time when you disturb the stick. Phugoids come in three flavors:

  1. Stable (Tending to stay at a fixed amplitude.)
  2. Unstable (Tending to get larger and larger as time goes on. Eventually resulting in a loss of control if the pilot doesn't intervene.)
  3. Damped (Tending towards smooth horizontal flight on its own.)

Passenger planes should be designed with stable or damped phugoids. This makes for a smooth ride. Fighters will often have unstable phugoids. This allows the pilot to change their attitude quite rapidly without the plane trying to seek straight and level.