A loss in well being or utility brought on by something that is not efficient. For example, is Person A steals Person B's bicycle, that is not a deadweight loss, since there the total number of bicycles is the same.
However if in the course of stealing the bicycle Person A destroys the bicycle, that would be a deadweight lost since there is now one few bicycle between them.
See also Deadweight Loss of Christmas and Excess Burden.
This bit of jargon has been viewed 9,441 times.