A choice can have a cost even when no money changes hands. Opportunity cost gives a name to the value of the best alternative that the decision leaves unavailable.
Name a real alternative
The St. Louis Fed’s economics framework defines the concept through the best alternative forgone. It does not ask you to add together every imaginable thing you could have done instead.
In an invented example, one free afternoon could go to a reading group or a walk with a friend. If those are the genuine competing choices, choosing one gives up the other. A third activity you would never have selected is not automatically the relevant comparison.
Keep the value personal and explicit
The value need not be a market price. Time with a friend and a learning experience may matter in ways that a money figure would describe poorly.
When a financial author invokes opportunity cost, ask whether the alternative is realistic and available to the person in the example. Write it beside the chosen action. The concept becomes useful when it clarifies an actual trade-off, rather than making every ordinary decision carry the weight of an unlimited list of possibilities.