In economics, what type of demand curve is horizontal at a market price?

In economics, a horizontal demand curve at a market price represents perfectly elastic demand.

Perfectly elastic demand means buyers will purchase any amount offered at one particular price, but none at a higher price. Graphically, the curve is horizontal, so an extremely small price increase reduces quantity demanded to zero in the idealized model. The price elasticity of demand is infinite.

This concept is mainly a theoretical benchmark. An individual firm in a perfectly competitive market is often modeled as facing perfectly elastic demand because it can sell at the market price but cannot charge more than rival firms selling an identical product.

Perfectly elastic demand is the opposite of perfectly inelastic demand, shown by a vertical curve. With perfectly inelastic demand, quantity stays fixed regardless of price. Real-world demand is rarely perfectly elastic, but the model helps explain why competitive firms are price takers.