1. The problem asks: What is the definition of opportunity cost? 2. Opportunity cost is a concept in economics that refers to the value of the next best alternative that is foregone when a decision is made. 3. It is not a past cost or a cost recorded in accounting books, but rather the benefit you miss out on by choosing one option over another. 4. It is never always zero because choosing one alternative always means giving up another. 5. Therefore, the correct answer is: The benefit foregone from choosing one alternative.