Opportunity cost
the value of the next best alternative that must be given up because of that decision (for example, working instead of going to school)
Abstraction
ignoring many details so as to focus on the most important aspects of a problem
Theory
a deliberate simplification of relationships used to explain how those relationships work
Correlation
the relationship between two variables that tend to go up or down together; does not imply causation
Economic model
a simplified, small-scale version of some aspect of the economy.; often expressed in equations, by graphs, or in words