Study Claims Iraq's 'Excess' Death Toll Has Reached 655,000
Excess deaths generally means over and above those that would have died anyway - the normal mortality rate for that population. The assumption being that those are the deaths you can blame on the war.
This is not a common usage. For once the journalists make it relatively clear what they mean in the body of the article; the number of deaths in Iraq in the last year, as estimated by this study, that exceed the official estimates.