Deadweight Loss

The market allows the effective rationing of scarce resources through self-interested profit incentive. When a price is determined through the dynamic interplay of supply and demand - that price is an aggregate. There are some buyers that will regard it as a bargain since they would have been prepared to pay more, but since it is available at a lesser price they transact at that level (that is lower than their subjective valuation). For them, when the purchase the product they gain - the aggregate of all those that gain is called a "Consumer Surplus". In the same way there are producers that would have been prepared to sell their wares below the market clearing price. These producers gain value over and above their own subjective assessment of the value - this is called a producer surplus. Most textbooks will give you a graphical illustration of this.

Here is a nice graphic on Wikipedia

so,

Definition of Consumer surplus:

Where some consumers would have been prepared to pay a higher price. The total consumer surplus is the quantity of consumers at every price level multiplied by the price level that they would still be prepared to transact.

Definition of Producer surplus:

The producers that would have been prepared to sell at a lower price.

When there is an intervention in the market however and normal market forces are not allowed to operate freely (as one finds in fascist states where the government uses force to control its citizens and by definition the market, both the consumer and the producer may lose value that they otherwise might have gained - these losses are referred to as deadweight loss.

Definition of Deadweight loss:

Values that could have been attained had it not been for the intervention. It is the degree to which the market is not allowed to operate optimally.