deadweight loss
Think of two people who would both gladly trade — a buyer who'd happily pay 8 dollars for a haircut, and a barber happy to give one for 5. There is value waiting to be created: a deal at 6 dollars would leave both better off. Now suppose a rule, a tax, or a monopoly's price-setting stops that haircut from ever happening. The gain that both would have enjoyed simply evaporates — it doesn't go to anyone, it just disappears. That vanished value is deadweight loss.
Precisely, deadweight loss is the reduction in total surplus that occurs when a market produces less (or more) than the efficient quantity — when output strays from where price equals marginal cost. It is 'dead weight' because, unlike a tax that transfers money from buyers to the government, this loss is gain that nobody captures: the trades that would have benefited both sides never take place. On a supply-and-demand diagram it appears as a triangle wedged between the demand and supply curves over the missing units, which is why it is often nicknamed the 'welfare triangle'.
Deadweight loss is the price tag economists attach to inefficiency. Taxes, subsidies, price ceilings, price floors, tariffs, monopoly pricing, and unaddressed externalities all create it, and measuring its size is how policymakers weigh the cost of an intervention against its benefits. The honest nuance: a deadweight loss is not always bad. A tax on cigarettes 'destroys' some trades on purpose, and if those trades caused harm to others, preventing them can actually raise true social welfare. The triangle measures lost private gains, which must be balanced against any external costs or benefits.
A new 4-dollar tax on coffee raises the price enough that some people who valued a cup at 3.50 above its cost simply stop buying. The tax collects revenue on the cups still sold, but the lost pleasure of those abandoned cups, captured by no one, is the deadweight loss.
The trades that never happen — value lost to everyone.
A tax's revenue is a transfer, not a loss — it goes to the government. The deadweight loss is only the extra harm: the mutually beneficial trades the tax discourages, which benefit nobody.