What is Opportunity Cost?

Legal Definition
In microeconomic theory, the opportunity cost, also known as alternative cost, is the value (not a benefit) of the choice of a best alternative lost while making a decision. A choice needs to be made between several mutually exclusive alternatives. Assuming the best choice is made, it is the "cost" incurred by not enjoying the benefit that would have been had by taking the second best available choice. The New Oxford American Dictionary defines it as "the loss of potential gain from other alternatives when one alternative is chosen." Opportunity cost is a key concept in economics, and has been described as expressing "the basic relationship between scarcity and choice." The notion of opportunity cost plays a crucial part in attempts to ensure that scarce resources are used efficiently. Thus, opportunity costs are not restricted to monetary or financial costs: the real cost of output forgone, lost time, pleasure or any other benefit that provides utility should also be considered an opportunity cost.
-- Wikipedia
Legal Definition
The cost of using a resource to acquire one thing instead of another. As all resources can be used in multiple ways, using them in one way invariably involves some opportunity cost.