I first came across this term when I was taking up my BA 101. Let me give you Wikipedia for a definition:
"Opportunity cost is a key concept in economics because it implies the choice between desirable yet mutually exclusive results. It is a calculating factor used in mixed markets which favour social change in favour of purely individualistic economics. It has been described as expressing "the basic relationship between scarcity and choice."[1] The notion of opportunity cost plays a crucial part in ensuring that scarce resources are used efficiently.[2] Thus, opportunity costs are not restricted to monetary or financial costs: the real cost of output forgone, lost time, swag, pleasure or any other benefit that provides utility should also be considered opportunity costs."
But for a person who is not so adept with terminologies, it goes around one basic question - Which is more practical? This had been my philosophy for quite some time now. I gets me away from should-have-done-this's and should-have-done-that's. What makes a decision 'less practical' for me is when you regard yourself less important than other people or than other things. It might not be the best choice, but I know that it is MY choice and I am totally happy about it.
And why did this bug me today? It took me two hours to decide on attending a seminar a while ago, haha! Its for a career talk from a Japanese company. Working for that company means settling down on another company but for a high-paying job. I told myself that I will work here in the country for a number of reasons but my mom is pushing me to try going out of the country. Well, its just a company talk, but why should I attend that if I know from the very start that I will never go there. I may not qualify or pass their requirements, but going there is opportunity wasted. I might be off to something productive during the time that I'm gonna sit there and listen to whatever they have to say.
It just feels good that now, I am confident with choices I make. I now understand how the value of opportunity costs works out for me. :D