Hi aspirant,

If we try to understand Periodic motion in simplest words it can be defined as the motion that repeats itself regularly at fixed intervals of time. It is path independent and time intervals may range from seconds to years. For example: a car moving on a straight road is said to have linear motion, the motion of the earth around the sun is circular motion, moving of pendulum is a harmonic motion.

Harmonic motion is just a special case of Periodic motion which is , repetitive movement back and forth through an equilibrium, or central, position, so that the maximum displacement on one side of this position is equal to the maximum displacement on the other side.

Note- Harmonic motion refers to simple harmonic motion only unless it's type is defined.

Hope it helps!!