Oscillatory motion that i) has a constant amplitude, ii) sinusoidal displacement function, and iii) has a restoring force acting directly proportional to the displacement from equilibrium position and is directed towards it. In essence, it is a oscillatory motion that has non-constant velocity (apart from being a sinusoidal func).
E.g., a simple pendulum (angular motion), a mass-spring system (linear motion).
Constraints on SHM,
3 & 4 -> &
Forces impacting SHM: