Deadweight loss
Deadweight loss is the loss of total welfare that arises when a market is prevented from reaching its efficient outcome.
Some value simply vanishes, gained by no one, when a market is prevented from doing what it would otherwise do. Economists call the waste deadweight loss.
Deadweight loss is the loss of total welfare that occurs when a market is kept from reaching its efficient outcome, so that mutually beneficial trades fail to happen. It is value that nobody captures, neither buyers, nor sellers, nor the government, simply forgone.
Trades that never happen
The key to deadweight loss is the transactions that do not take place. When a tax, price control, monopoly, or other distortion drives a wedge between what buyers would pay and what sellers would accept, some trades that would have benefited both sides no longer occur. The surplus those trades would have created is lost to everyone. That forgone surplus, not the money transferred from one party to another, is the deadweight loss.
Tax as the standard example
A tax illustrates it cleanly. Part of the tax simply transfers money from buyers and sellers to the government, which is not a loss to society, just a redistribution. But the tax also discourages some trades that were worth doing, and the value of those lost trades is pure waste. This is why economists judge taxes partly by how much deadweight loss they cause: a tax on something people can easily stop buying causes more waste than a tax on something with inelastic demand.
The cost of interference, and its justification
Deadweight loss is the standard way of measuring the efficiency cost of interfering with a market, whether through taxes, subsidies, price ceilings, or monopoly. That does not make all interference wrong; a tax may fund something worthwhile, and a market left alone may produce outcomes society rejects. But deadweight loss puts a price on the distortion, so the benefits of intervening can be weighed against the value quietly destroyed.
Deadweight loss is one of the more sobering ideas in economics, because it describes value that does not change hands but simply disappears. Recognising it keeps the focus on a question intuition misses: not just who gains and who pays, but how much is lost that no one gets at all.