The federal Social Security Government Pension Offset (GPO) is the method Social Security uses to determine the level of Social Security survivors benefits for retirees who receive a pension from an entity which did not participate in the Social Security System.
http://www.ssa.gov/planners/retire/gpo.html
https://www.ssa.gov/pubs/EN-05-10007.pdf
https://www.ssa.gov/benefits/retirement/planner/gpo-calc.html