measurement of the degree of flexibility of something in response to something else. For example, the "elasticity of demand with respect to income" or the "income elasticity of demand" for a product refers to the percentage change in the quantity of the product demanded in response to a 1% change in consumers' income, or more generally to the ratio of the percentage change in quantity demanded to the percentage change in income. The change in the denominator always causes the change in the numerator, so the elasticity can be said to be the ratio of a percentage change that is caused to the percentage change of something that is causative. "Rational" In general usage, one is said to be rational if one is sane or lucid. In economics, rationality means that an economic agent specifies, or acts as if he implicitly specifies, a way to characterize his or someone's well-being, and then takes into account all relevant information in making choices so as to optimize that well-being. For example, an individual consumer is assumed to be rational in the sense that he maximizes a utility function, which expresses his subjective sense of well-being as a function of the amounts of various goods he consumes; firms are assumed to maximize profit or some related goal. Economists assume that in the presence of uncertainty, an agent is rational in the sense of specifying a way of evaluating sets of possible outcomes (and associated probabilities) with some function: A consumer is assumed to choose his consumption levels of various goods so as to pick the set of possible outcomes, and associated probabilities, that maximizes this function, which is often assumed to be the expected value of a von Neumann–Morgenstern utility function; a firm is often assumed to maximize the expected value of profit. "Rent" In general usage, rent refers to a payment made in exchange for temporary use of property, for example paying rent to stay in an apartment. In economics, rent is any payment to an owner or factor of production in excess of the costs needed to bring that factor into production. Effectively, it is payment made to a producer above and beyond what would have been necessary to incentivize them to produce. It can roughly be understood as unearned revenue. In many cases, common-usage rent is an example of economic-usage rent, making the distinction between the two confusing. References Technical