In microeconomic theory, the opportunity cost, also known as alternative cost, is the value (not a benefit) of the choice of a best alternative cost 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."