Opportunity cost

Opportunity cost is the value of the next best thing you give up whenever you make a decision. It is "the loss of potential gain from other alternatives when one alternative is chosen". 
The idea of an opportunity cost was first begun by John Stuart Mill. The utility has to be more than the opportunity cost for it to be a good choice in economics.
For example, opportunity cost is how much leisure time we give up to work. Because leisure and income are both valued, we have to decide whether to work, or do what we want. Going to work implies more income but less leisure. Staying at home is more leisure yet less income.
Another example is deciding to stop work to go back to university. By choosing to go to university, your opportunity cost is losing your job and your pay check. Even though going back to university has a big opportunity cost, many people think it is a good decision because increased education gives you more job opportunities.
And in other words, opportunity cost can be termed as cost of forgone alternatives or cost of shifting from one opportunity to the other.