A retired teacher with severe depressive illness, who in reality has a comfortable pension and substantial savings, becomes utterly convinced she and her family are about to be made destitute, refusing to spend money even on basic food and remaining unmoved when shown her own bank statements. What is this?

This is a delusion of poverty: a fixed, false belief in financial ruin or impending destitution that is objectively untrue, here unresponsive even to her own bank statements, and classically associated with severe depressive illness. It is not a delusion of guilt, which concerns moral wrongdoing rather than financial ruin, though the two often occur together as part of the same depressive delusional constellation. It is not realistic financial worry, since by definition that would be proportionate to her actual circumstances and open to correction by evidence such as her bank statements — exactly what fails to happen here. And it is not a delusion of ill health, which concerns physical illness rather than money.