Opportunity Cost: The value of the next-best alternative that must be forgone in order to undertake an activity.
Opportunity cost of a production or consumption choice = The value of the next best option
If you have 100 Rs and with this 100 Rs you can either buy a shirt or watch a movie. If you will watch the movie then the opportunity cost of watching the movie is the value of shirt that you would have bought in place of movie.
Opportunity cost in terms of PPC
Sacrifice of commodity X from point B to point C to get extra units of commodity Y from D to E.
Sacrifice of commodity X from point B to point C = Opportunity Cost to get ed units of good y.