This person is no longer participating in the labour market and so is not counted as unemployed. But he or she could still be pulled back into the workforce if the right opportunity came along and so represents a potential source of new supply.
Governments or their central banks may need such an anchor to stabilize inflation, and they can generate the anchor by announcing an inflation target and then doing what they must do to hit that target. Such an approach would also lead to more transparent monetary policies, as economic actors would better understand the goals of monetary authorities.