Supply and Demand
A classic economic model of price determination in a market. In a competitive market, the unit price for a particular good or other traded item such as labor or financial assets, will vary until it settles at a point where the quantity demanded will equal the quantity supplied, resulting in an economic equilibrium for price and quantity transacted.
Origin
Scottish political economist James Denham-Steuart first paired the terms in his 1767 Inquiry into the Principles of Political Economy. Adam Smith adopted the framing in The Wealth of Nations (1776), and Alfred Marshall formalized the familiar intersecting-curves diagram in his 1890 Principles of Economics — synthesizing demand curves first sketched by Augustin Cournot in 1838 and supply curves added by Fleeming Jenkin in 1870. Marshall's "scissors" metaphor, with the two curves meeting at equilibrium, became the enduring visual for the model.