Every choice costs something, even when no money changes hands. Opportunity cost is the value of the best alternative you give up when you choose one option over another.

This idea is powerful because it forces clearer thinking. Time spent on one project cannot be spent on another. Money used for one purchase cannot fund a different goal. Opportunity cost turns decision-making into an exercise in tradeoffs rather than wishful thinking.

Reference:

Wikipedia: Opportunity cost

https://en.wikipedia.org/wiki/Opportunity_cost