The value of the next-best alternative that is given up when a choice is made. In economics, every "yes" is also a "no" to every other option that could have been pursued with the same resources.
Opportunity cost is the most under-weighted concept in business decisions because the foregone alternatives are invisible — there is no line item for the deal you didn't do or the hire you didn't make. Decision quality improves dramatically when teams force themselves to name the second-best option before committing to the first.