My great-grandmother had a mental illness: she imagined someone standing at the door with an axe, etc.; believed in spurious events ("you stole my potatoes"). What was that? It was dementia (acquired dementia). Frequently it is followed by psychotic symptoms — delusions (mostly simple, unsystematized, limited to mundane issues, unlike schizophrenia, where delusions build up into a whole system), hallucinations (also basic — someone standing in the corner or outside the window, a cat running around on the floor)