This document defines key terms from chapters 18, sections 1 and 2 of an economics textbook. In section 1, it defines needs as things required for survival, wants as things we would like to have, economics as the study of how we make decisions with limited resources, microeconomics as looking at small economic pictures, and macroeconomics as looking at the big economic picture. It also defines economic models, economic systems, resources, and scarcity. Section 2 defines trade-offs as alternatives when choosing one thing over another, opportunity cost as the cost of the next best alternative, marginal cost as the additional cost of one more unit of output, marginal benefit as the additional satisfaction from one more unit, and cost-benefit analysis as