(1) Periodic motion : A motion that repeats itself at definite intervals of time is said to be a periodic motion.
Examples: The motion of the hands of a clock, the motion of the Earth around the Sun.
(2) Oscillatory motion : A periodic motion in which a body moves back and forth over the same path, straight or curved, between alternate extremes is said to be an oscillatory motion. Examples: The motion of a taut string when plucked, the vibrations of the atoms in a molecule, the oscillations of a simple pendulum.
[Note: The oscillatory motion of a particle is also called a harmonic motion when its position, velocity and acceleration can be expressed in terms of a periodic, sinusoidal functions-sine or cosine, of time.