[[Oscillations]]
It's when a restoring force is proportional to the displacement from equilibrium.

# Summary
* $F_x=-k\Delta x$
* Acceleration always points opposite to displacement.
* Equilibrium is where net force is zero.
* Maximum displacement is amplitude.
* Velocity is maximum at equilibrium.
* Pendulum acts as SHM at low angles because restoring force is proportional to angle.