Is opportunity cost an ambiguous and arbitrary concept or a simple, straightforward, and fruitful one? This reexamination of opportunity cost addresses this question, and shows that opportunity cost is an ambiguous concept because "two" definitions are in widespread use. One of the definitions is indeed simple, fruitful, and one that students can learn. The other has the potential to be ambiguous. The widespread use of two definitions of a fundamental idea raises a second question: Do we have two "conflicting" versions of opportunity cost, or are the two versions "equivalent"? This article shows that in some situations the two versions are equivalent, but in others they are not. It is when they are not equivalent that there is the greatest scope for ambiguity. "Nothing is important about a definition. It is only useful insofar as it helps us to think about a problem and to make the right decisions" (Ferraro and Taylor 2005). This statement raises a third question: What are the problems and decisions for which we use the opportunity cost concept, and which of the two versions does the better job at addressing those problems? This article provides the origins and definitions of the two versions of opportunity cost, provides examples of situations in which the two versions of opportunity cost are equivalent and not equivalent, and discusses the purpose of opportunity cost.