What concept describes the next best alternative foregone in a decision?

Opportunity cost is the value of the next-best alternative foregone when a choice is made.

The idea follows from scarcity: time, money, labor, land, and other resources have competing uses. Choosing one option means giving up the benefits that the best rejected option would have provided. The cost is therefore not necessarily a cash payment; it can be lost time, income, output, leisure, or satisfaction.

Opportunity cost is often confused with sunk cost, which is a past cost that cannot be recovered, or marginal cost, which is the additional cost of producing one more unit. Transaction costs are the expenses of making or enforcing an exchange, such as searching for information or negotiating a contract.

The concept also underpins comparative advantage. A person, firm, or country should generally specialize in activities it can perform at a lower opportunity cost, even if another producer is more efficient in absolute terms. The “next best” qualification matters: the relevant comparison is the most valuable alternative actually available, not every option that was rejected.