Opportunity Cost

Opportunity cost is the cost of an alternative that must be forgone in order to pursue a certain action. Put another way, the benefits you could have received by taking an alternative action.

Read more: http://www.investopedia.com/terms/o/opportunitycost.asp#ixzz2HqeVhZRz

OR the simple definition is that opportunity is basically when we are giving up to do something else.

Example:

In terms of activities:

Let's just say you get to choose either you want to watch a movie at a mall OR shopping? And you chose to go shopping, well, means that watching movie at the mall is your opportunity costs, since you chose shopping instead of watching movie. 

In terms of economics:

Let's just say that you invested in a company that produces wood furniture for about 4%. Then after a year you started investing in another company except that it actually produces metal furniture. And you invested in it for about 8%. So, when you deducted the investment that you had made for the wood company, from the investment for the metal company, you will get 4% left. (8%-4%=4%) Basically, your opportunity costs would be the 4% left.