Economics is based on the idea of scarcity and choice. When you make a choice, you have decision or item you chose and the one you did not choose. Economists assign a value to the decision you did not choose it is called opportunity cost.
Opportunity cost is the value of the next best alternative.
It is not the value of all alternatives. Opportunity cost is also known as social cost. Opportunity cost is subjective. Opportunity cost depends on the alternatives and varies by situation. Sunk cost a cost that must be incurred no matter what.