I read about two motions of objects
Periodic motion &
Are they same motion? If not what is the basic difference between these two motions?
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In simple words,
A periodic motion is one which repeats itself after certain period of time
Example: Motion of Earth around the sun is a periodic motion
An oscillatory motion is to and fro motion about a mean position.
Example: Motion of pendulum (which is also a SHM)
The second motion is subset of first one i.e. each oscillatory motion is a periodic motion but vice versa is not true.
I would add to above answer, that typically a oscillation happens, when energy is transformed from one to another form and vice versa and therefore keeping the motion running.
For a periodic movement the most important is the periodic behaviour in time. Therefore this "definition" has other restrictions than the former.