The quality-adjusted life year (QALY) is a measurement used in adjusting measures of life expectancy and effectiveness of medical interventions.
As the name indicates, this is not just a measure of life expectancy, but of the quality of life expected. Each extra year of life is 'discounted' in subjective value based on the amount a person is expected to suffer; 20 years of very painful, bedridden life might be considered less valuable than 15 years of relatively pain-free, mobile life. Some, although not all, QALY models allow for negative values, in which the level of suffering is so high that death is preferable.
This has become a point of contention when QALY is used to determine whether or not (or how) a patient will be treated. In United Kingdom the National Health Service uses "£ per QALY" as a factor in evaluating interventions; for example, and with a number of caveats, if a cancer drug costs more than £30,000 per expected QALY it is normally not approved.
QALYs are also used in the area of humanitarian interventions. For example, if you donate just enough food to keep a starving person alive, then you are doing a good thing -- saving their life -- but you are also leaving them to live in a constant state of hunger and poverty. Their life expectancy has improved a good bit, but their quality of life has not improved very much. Donating enough food to feed them well and medicine to help them stay healthy will improve their QALY by quite a bit more. Humanitarian organizations may use a wider measure, the well-being adjusted life year (WALY), which takes other factors than health into account.