at a range of different prices. If, for example, a rise in income causes a consumer to be willing to purchase more of a good than before contingent on each possible price, economists say that the income rise has caused the consumer's demand for the good to rise. In contrast, if a change in market conditions leads to a decline in the price of a good resulting in a consumer's being willing to buy more of it, economists say that the consumer's quantity demanded of the good has risen. A change in quantity demanded is represented by a movement along the demand curve, while a change in demand is represented by a shift of the demand curve. In popular usage a change in "demand" can refer to either what economists call a change in demand or what economists call a change in quantity demanded. "Supply" In economics, supply refers to the strength of one or many producers' willingness to produce and sell a good or goods at any in a range of prices. If, for example, a reduction in production costs causes a producer to be willing to provide more of a good than before contingent on each possible price, economists say that the drop in production costs has caused supply to rise. In contrast, if a change in market conditions leads to a decline in the price of a good resulting in a producer willing to sell less of it, economists say that the consumer's quantity supplied of the good has fallen. A change in quantity supplied is represented by a movement along the supply curve, while a change in supply is represented by a shift of the supply curve. "Marginal" While "marginal" in common usage tends to mean tangential, implying limited importance, in economics "marginal" means "incremental". For example, the marginal propensity to consume refers to the incremental tendency to spend income on consumer goods: the fraction of any additional income which is spent on additional consumption (or conversely, the fraction of any decrease in income which becomes a decrease in consumption). Likewise, the marginal product of capital refers to the additional production of output that results from using an additional unit of physical capital (machinery, etc.). If very small increments are being considered, so that calculus is used, then this ratio of incremental amounts is a derivative (for example, the marginal propensity to consume becomes the derivative