Opportunity cost

Opportunity cost is an economic concept that means "the value of the next best alternative". The best way to explain it is with a few examples:

1. If I quit my job and spend a year travelling, the direct cost is whatever I spend on plane tickets, food and accommodation while travelling. The opportunity cost is that direct cost plus the year's salary foregone - because if I hadn't gone travelling, not only would I have saved the money, I would have earned a salary as well.
2. If I buy a new pair of shoes, the direct cost is what I paid for them. The opportunity cost is the value of the next best thing I could have done with that money.