The cost of choosing one course of action over another. Letting your second best option
pass.
Economists tend to measure opportunity cost in terms of
money for simplicity's sake. However value isn't
always monetary.
At the restaurant, you are presented with a desert menu. Assuming that you only
eat one desert, you have the choice between chocolate mouse, ice cream and honey crepes. Let's say you prefer ice cream to chocolate mouse, but honey crepes are your favorite. If you eat a honey crepe, ice cream (which would have chosen had you not had crepes) is your opportunity cost.
Another example would be if two women that you fancied,
Sarah and
Alice, asked you out and you said yes to
Sarah (assuming monogamy), going out with Alice is your opportunity cost.