In economics, what is the loss of total surplus caused by a market inefficiency called?

In economics, the loss of total surplus caused by a market inefficiency is called deadweight loss.

Deadweight loss represents mutually beneficial trades that do not occur. In the standard supply-and-demand model, total surplus is the sum of consumer surplus and producer surplus. A tax, binding price control, monopoly restriction, or other intervention can reduce the number of trades below the efficient level, creating a loss that is not received by buyers, sellers, or the government.

A common diagram shows deadweight loss as a triangular area between the demand and supply curves over the units that are no longer traded. The exact shape depends on the curves and the intervention. Tax revenue, for example, may transfer surplus to the government, but the lost trades create the separate deadweight loss.

Deadweight loss is a model-based measure, not simply any fall in private income. Redistribution can change who receives surplus without necessarily creating deadweight loss, while a policy may deliberately accept some efficiency loss to pursue equity, health, environmental, or other goals.