Wastage

n. decay, deterioration; wasting; lessening; waste

Wastage was a British term used during World War I. It was used to describe the severity of losses experienced during the war. On the quietest day of the war the British were losing 7,000 men killed and wounded.In retail, the term is used to refer to damaged, out of date, reduced, or generally unsaleable items which are destined to be thrown away and which are written off as a loss.
See more at Wikipedia.org...

Noun
1. the process of wasting
(hypernym) decrease, decrement
(derivation) pine away, waste, languish
2. anything lost by wear or waste
(hypernym) loss

(n.)
Loss by use, decay, evaporation, leakage, or the like; waste.
  

in advertising, that part of the audience or readership of a media vehicle which is "wasted" because it is not part of the target market.