Aircraft stability

Stability is the aircraft's tendency to return to trimmed flight after a disturbance.

Static stability: the initial tendency to return to trim.
Dynamic stability: what the motion does over time: damped (settles down), oscillating, or divergent (gets worse).

(The static/dynamic split is basic aerodynamics; TP 1102 defines stability as the tendency to return to trim and covers the three axes, but does not use these two terms.)

Each axis gets its stability from a different place:

Stability	Motion	Comes from
Longitudinal	pitch	horizontal stabilizer + CG position
Lateral	roll	wing dihedral
Directional	yaw	vertical fin

Aircraft roll, pitch and yaw axes through the centre of gravity — The three axes: pitch (longitudinal stability), roll (lateral), yaw (directional). NASA, public domain, via Wikimedia Commons.

CG position sets longitudinal stability:

Forward CG: more stable, but higher stall speed and more elevator needed to flare (the tail carries more download).
Aft CG: less stable, lighter in pitch.

Trap: thinking a forward CG lowers the stall speed. It raises it.

References

TP 1102 (Flight Training Manual)

PPL Tester

Aircraft stability

References

Practice