Wiktionary
n. (context economics English) The cost of an opportunity forgone (and the loss of the benefits that could be received from that opportunity); the most valuable forgone alternative.
WordNet
n. cost in terms of foregone alternatives
Wikipedia
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 an opportunity cost.
Usage examples of "opportunity cost".
The question becomes how much overlap or redundancy between land, sea, air, and space forces can the nation afford, and what is the opportunity cost to the core competency of the land, sea, air, or space force that builds and/or maintains the duplicative force structure.
The opportunity cost of something is measured as the sum of all the other things that you have to forgo in order to have that something.
Hence a large penis's cost should be regarded as a lost-opportunity cost: because any man's available biosyn-thetic energy is finite, the energy squandered on one structure comes at the expense of energy potentially available for another structure.