A measure of the welfare that is lost when the equilibrium in a market is not Pareto efficient. Deadweight loss arises, for example, when a monopoly sets a price above marginal cost or when a government levies a commodity tax. In the case of monopoly some of the ^consumer surplus that would be present if the market were competitive is captured as monopoly profit but some is lost; the part that is lost is the deadweight loss. Similarly, a commodity tax captures some consumer surplus and *producer surplus as government tax revenue but some of each is lost.
Reference: Oxford Press Dictonary of Economics, 5th edt.