PM - Post Meridian, in Latin (After Noon, in English)
Post Meridian is used to represent different times of the day from 12 o'clock in the noon to 11:59 in the night (just before midday). Like, in a sentence we can write - "I will teach you from 2:00 PM" it means, i will teach you from 2 o'clock in the after noon.
PM - Phase modulation
Phase modulation is a technique in which, information is sent by varying the phase of a carrier wave. The phase shift can be of any unit like -90 degree or +90 degree.
PM - Prime Minister
The Prime Minister of India is the presiding and actual head of the government, chief advisor to the President of India and the leader of the majority party in parliament.