Question

why is the oscillatory motiona periodic motion?

Answer 1

periodic motion is a motion that repeat it's motion after a period..
while osciallatry motion is the to and fro motion like motion of humans heart

Answer 2

Periodic motion is that motion which repeats itself after equal intervals of time. ... For example, the motion of planets around the Sun is always periodic but not oscillatory. The motion of the pendulum of a clock is periodic as well as oscillatory