1. Defined as the benefit foregone by using a resource for one purpose instead of its next best alternative. Typically applied to capital and labor inputs to reflect their real costs to society as against their costs to a private entrepreneur which may be lower or higher because of subsidies, taxes, and various kinds of market distortions; 2. An amount a fisherman could earn for his time and investment in another business or occupation; 3. The benefits forgone by undertaking one activity instead of another.