When the price of your morning coffee jumps from $2 to$6, you probably stop buying it every day and switch to weekends only. That shift in how much you buy at different prices is demand in action. Demand is not a single number — it is the whole relationship between price and quantity: at each possible price, how much would buyers purchase? The pattern is consistent enough that economists call it the law of demand: as price rises, quantity demanded falls, and vice versa.

Formally, demand describes the relationship between price and quantity demanded holding everything else constant — a condition called ceteris paribus (Latin for 'all else equal'). On a graph, demand is a downward-sloping curve. It is important to separate two things that look similar but are not. A movement along the curve happens when the good's own price changes — you slide up or down the existing curve. A shift of the curve happens when something other than the good's own price changes: your income rises, your tastes change, the price of a substitute or complement moves, or your expectations about the future shift. A shift means the whole curve moves left or right. See how demand interacts with supply to produce a market equilibrium, or drag a demand shifter in the interactive supply and demand model to watch the curve move in real time.