机会成本（Opportunity Cost, OC）是指决策过程中面临多项选择，当中被放弃而价值最高的选择（highest-valued option forgone），又称为「替代性成本 (alternative cost)」，就是俗语的「世上没有免费午餐」、鱼与熊掌不可兼得。例如某甲现在有1小时时间，可挑选A、B、C三件同样都需要花费1小时才能完成的三件事之一来做，他若选择了A，就得放弃做B、C二事的机会；若选择了B，就得放弃做A、C两事的机会。又如在选择了某项社会福利（如创业津贴）后，受到固定资源的限制，他便得失去选择其他福利（如选择国民住宅）的机会──则放弃选择国民住宅福利的机会，即为选择创业津贴福利的「机会成本」。

In microeconomic theory, the opportunity cost of a choice is the value of the best alternative forgone where, given limited resources, 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 opportunity costs.