My elderly Mom had a severe stroke 4 years ago. Her Neurologist said this is not dementia. What is the difference?

Maybe her doctor meant she didn't have Alzheimer's, which would most likely be true. From the little you wrote, we can't tell if it might be vascular dementia. I know a man who has had several serious strokes. He is strange acting, but it isn't dementia. Dementia usually is diagnosed if there is memory loss and loss of reasoning. Dementia is really just a symptom, so a doctor saying there is no symptom when it's obvious is just being confusing. A caregiver is probably the first to know when there is dementia.

I'm not a medical expert, but there are a number of articles that I have read online about Stroke and Vascular Dementia. I think that strokes can cause Vascular Dementia, but they may occur in a patient and not cause dementia too. If the Neurologist can't offer any diagnosis regarding your mother's mental confusion and inability to know what is real, then I would get a second opinion...probably another Neurologist and a Geriatric Psychiatrist.