A "Hunting" Tendency
Stability - static and/or dynamic. In the real world most planes have a tendency to hunt. If you let go of your planes controls it won't fly at a constant speed.
Instead, it oscillates up and down. Why?
The up and down flight path of an oscillating plane is an complicated interaction of speed changes, flight paths, directions of the relative wind, angles of attack, changing drags, changing propeller efficiencies, horsepower outputs, changing lifts and slight downward and upward g-loads.
All the above means the plane is trying to get back to its proper cruising speed and proper angle of attack. The plane "over controls" itself and doesn't quite succeed in steadying itself down after you released the controls.
Since your plane is trying to return to a proper cruising speed and angle of attack, the apparent unstable "hunting" doesn't mean the airplane itself is unstable.
If the airplane was actually unstable, in a dive it would continue to dive or it would stay in a climb until it stalled.
The plane is primarily stable. If an airplane can't steady itself it is dynamically unstable. The airplanes oscillations continue to increase in violence until the plane disintegrates.
The majority of airplanes are dynamically stable where the oscillations become weaker and weaker. The presence of unstable air or other disturbances may prevent the dampening of the oscillations.
Knowing the technical theory on "hunting" is not very important to a pilot. Stability is important to a pilot because of the way it affects the feel and behavior of the plane while he controls the plane.
If the pilot is controlling the plane it will not oscillate.
